What are the possible causes of pettiness? There is a question about pettiness Is it possible to quantify &#39;pettiness&#39; in a personality? 

Which gives a good example of how some people can focus on seemingly unimportant issues, when there are more obvious issues that would seem to warrant attention.

What would cause a person to focus, even fixate on small, petty things; for example: 


a co-worker's tardiness, when the employer's company is facing a takeover and jobs may be lost.  
the makeup one of the school Mum's was wearing, when there's been a robbery at the school


I am wondering what the possible causes of pettiness may be?
There can always be many variables for what causes human behavior, within this answer I will focus on three more obvious or, perhap, likely causes for pettiness within individuals.
This could be one of a variety of causes stemming from the following:


Schadenfreude
Obsessive Compulsive Disorder (or related disorders)
Disorders on the Autistic spectrum


Schadenfreude:

In the majority of situations this would be the most likely cause of such pettiness, as it is applicable to a more mainstream type of behavior. The chance to show up a deficit or shortcoming in another, motivated by jealousy of, or a feeling of unease stemming from threat  by, that person..


  We are primed for pettiness, programmed to notice seemingly inconsequential gradations, but for good reason: Being chronically dissatisfied is an effective stimulus to best your more complacent peers. (1)


This could well be the result of schadenfreude (translated to harm-joy), which refers to the pleasure people gain from the misfortunes of someone they perceive to be a high achiever or be envious of. This is based on envy derived from poor self esteem and/or lack of validation of that person's self esteem and a reaction to a perceived threat, from a fellow high-achiever.

The greater the  focus on seemingly petty things, the greater the individual's  psychic conflict or neediness and by this I refer to, the need for validation of self-worth and reaffirming their rank as a human being within a given community.  For such anxiety to cloud judgement to give a, seemingly, inconsequential issue, precedence over what seem to be more important issues. For this individual the need to validate themselves as a person is greater than national, global or even cosmic issues for which this individual has no control over, and which may have no immediate effect on that person's life.


  The present research shows that people with low self-esteem experience more schadenfreude toward the misfortune of a high- achiever than those with high self-esteem and that this relationship is mediated by the self-threat evoked by the high-achiever. Moreover, the indirect relationship between self-esteem and schadenfreude is contingent on an opportunity of self-affirmation. When no self-affirmation opportunity is available, low-self-esteem participants experience a stronger self-threat when confronted with a high-achiever, and this self-threat increases their schadenfreude. (2)



Obsessive Compulsive Disorder (OCD)


Although OCD is usually referring to the compulsive ritualised activities of an individual, as a result of an underlying obsession. If an observer with OCD, witness what may be regarded to most people as petty, but the observed behavior happens to cross into the area of obsession for that person, it could well be that person's attempt to alleviate their own anxiety that is triggered by the perceived aberrant behavior. An example being, observing someone leave the toilet cubicle and restroom without washing their hands.


  Obsessive-Compulsive Disorder, or OCD, is an anxiety disorder and is characterized by recurrent, unwanted thoughts (obsessions) and/or repetitive behaviors (compulsions). Repetitive behaviors such as hand-washing, counting or cleaning are often performed with the hope of preventing obsessive thoughts or making them go away. Performing these so-called "rituals," however, provides only temporary relief, and not performing them markedly increases anxiety



Disorders on the Autistic spectrum


People with disorders that lie on the autistic spectrum, can frequently have preoccupation or obsession with details, that may seem inconsequential to others, or even petty. As with OCD, this perceived pettiness could be an attempt to alleviate the individual's anxiety and to assist in making sense of the world, as this type of individual is frequently inflexible and views the world through a set of structured ideas, that do not easily facilitate or blend with the fluid nature of life and human behavior.


  “Obsessions”, “circumscribed interests”, “special interests”, “routines”, “rituals” , 
  “preoccupations” are some of the terms used when describing the behaviour of young 
  children with autism. These behaviours belong to one of the three core areas of 
  impairment in children with autism. (4)




References:


Why You Think You'll Never Stack Up by Carlin Flora, Psychology Today (1)
Self-Esteem, Self-Affirmation, and Schadenfreude
Wilco W. van Dijk, Guido M. van Koningsbruggen, Jaap W. Ouwerkerk and Yoka M. Wesseling DOI:  10.1037/a0026331 PDF (2)
Obsessive-Compulsive Disorder (OCD)
Brain and Behavior Research Foundation 
Restricted, repetitive and stereotyped patterns of behaviour, interest and activities
Dr Avril V Brereton, Monash University  PDF (4)