Why infant experiences shame as shyness when some unknown face looks at him/her? In the book &quot;Healing the shame that binds you&quot; by John Bradshaw (link to Google Books sample pages - scroll to page 10), there is a section about healthy shame and it's described how healthy shame looks like for different age groups/how it progresses or advances with age.

Shame as shyness
Once basic trust has been established, the child's feeling of shame emerges.  The first appearance of the feeling of shame usually occurs at about six months. At that age, a child has become familiar with his or her mother's face. When a strange face (maybe a relative seeing the baby for the first time) appears, the infant experiences shame as shyness in looking at the strange face.
Some children are temperamentally shy and withdrawn. But all of us experience some shyness in the presence of what is unfamiliar.

It's written that in age of like 6 months, infant is accustomed to primary caregiver's face and experiences shame as shyness when somebody unknown looks at infant and he/she sees this.
Why does infant experiences shame when some unknown face looks at him/her?
And why shyness is shame?
References
Bradshaw, J. (2005). Healing the Shame That Binds You (Expanded and Updated Edition): Recovery Classics Edition. Deerfield Beach, FL: Health Communications, Inc. ISBN 978-0-7573-0323-4
When looking at the whole idea you pointed out, it sounded preposterous.  How would a 6 month old infant have any concept of shame? As @BryanKrause pointed out in the comments, there are a lot of self-help books out there which are not based in the sciences of Psychology, but there are some good ones out there, such as Laura Davis' The Courage to Heal Workbook and John Bradshaw pioneered the concept of the &quot;Inner Child&quot; and brought the term &quot;dysfunctional family&quot; into the mainstream (Bradshaw, 1990) so I thought I would look into this.
John Bradshaw is a New York Times Bestselling Author and founding father of the self-help movement (John Bradshaw Media Group, n.d.).  Looking for his qualifications in Psychology, Wikipedia points out that:

He earned a B.A. degree in Sacred Theology and an M.A. degree in philosophy from the University of Toronto in Canada. Six years after his 1963 graduation, Bradshaw returned to academia at Rice University in Houston, Texas, doing three years of graduate work in psychology and religion.

Looking at the sample pages linked, the idea pointed out is in the first section of the book, which is titled The Healthy Faces of Shame (HDL Shame) with HDL liking the shame to the HDL (high-density lipoprotein) variety of cholesterol—the &quot;Good&quot; cholesterol.
As the quote pointed out is referring to a 6 month old infant, this is in the Interpersonal Bridge: Established Codependence stage of Bradshaw's Developmental Stages of Healthy (HDL) Shame (Page 9 of the book you linked).
Bradshaw's Development Stages of Healthy (HDL) Shame




Stage
Description




Interpersonal BridgeEstablished Codependence
6 MonthsOnce securely attached—shame as SHYNESS appears as a response to being exposed to strange faces




When you look at the various models of development stages or life stages, they can be a matter of opinion and there are 5 spectrum models of development which theorists – and people in general – often disagree.
Crozier (2010) points out that:
page 43

Research on shyness in the early years tends to draw upon observations of children's behaviour (e.g., Kagan, 2001), whereas studies in later childhood also draw upon self-report questionnaire and interview methods (e.g., Crozier &amp; Burnham, 1990; Crozier, 1995).

page 47

[T]he relations among shyness, shame, and embarrassment are little understood, and there is disagreement as to whether they are versions of the same underlying affect or emotion (Izard, 1977; Tomkins, 1963), or whether they constitute distinct emotional states (Keltner &amp; Buswell, 1997).

Reading this book, and maybe reading the cited texts within, highlights the reason there is so much disagreement and gives a more balanced view on the research behind shame and shyness.
Looking at 2 different theories of development of shyness pointed out in Crozier (2010), one of which is

Buss A.H. (1986) A Theory of Shyness. In: Jones W.H., Cheek J.M., Briggs S.R. (eds) Shyness. Emotions, Personality, and Psychotherapy. Boston, MA: Springer. https://doi.org/10.1007/978-1-4899-0525-3_4

for the idea that a 6 month old hides in shame when they are shy, I don't have children myself, but when observing my step-grandchildren when they were infants, to me it is still preposterous. As Buss (1986) points out, the shyness at this age is fearful shyness as they are fearful of someone who essentially is a stranger.

Fearful shyness starts during the first year of life, usually during the latter half of the year. Sometimes called stranger anxiety, the reaction occurs mainly when unfamiliar people, usually adults, confront the infant. The typical response is wariness, retreat, and the seeking of comfort in the security of mother's arms; more intense reactions include a cry-face or the crying and shrinking back that characterize fear.

The sense of shame isn't developed until they are (depending on advancement etc.) around 18 months old to 3 years old.
With the concept of &quot;lockdown babies&quot; (The Children's Commissioner, 2020), even though some parents may be introducing other members of family to their infants via Zoom, Skype etc. I would imagine that, along with other social issues, there will be more issues with fearful shyness with opening up outdoor spaces to everyone as the Covid crisis lessens.  Not just with strangers approaching, but also through exposure to so many people at once.
References
Bradshaw, J. (1990). Home Coming: Reclaiming and Championing Your Inner Child New York, NY: Bantam Books ISBN: 978-0-5530-5793-5
Bradshaw, J. (2005). Healing the Shame That Binds You (Expanded and Updated Edition): Recovery Classics Edition. Deerfield Beach, FL: Health Communications, Inc. ISBN 978-0-7573-0323-4
Buss A.H. (1986) A Theory of Shyness. In: Jones W.H., Cheek J.M., Briggs S.R. (eds) Shyness. Emotions, Personality, and Psychotherapy. Boston, MA: Springer. https://doi.org/10.1007/978-1-4899-0525-3_4
Crozier, W. R. (2010). Shyness and the development of embarrassment and the self-conscious emotions. In K. H. Rubin &amp; R. J. Coplan (Eds.), The development of shyness and social withdrawal (p. 42–63). New York, NY: The Guilford Press. 978-1-6062-3523-2
John Bradshaw Media Group. (n.d.). John Bradshaw https://www.johnbradshaw.com/
The Children's Commissioner (2020). Lockdown babies: Children born during the coronavirus crisis https://www.childrenscommissioner.gov.uk/report/lockdown-babies/