How to apply different motivational theories to understand career orientation?
Assuming the specific case of being "motivated towards a career" rather then simply getting a job to make ends meet, there can be several factors.

Social status is probably the most common pursuit of a career.

Enjoyment, which is the ideal reason for pursuing a career. Usually a career sought for enjoyment is referred to as 'pursuing one's dream'.

Proximity (a.k.a. convenience) is another driving factor. Which could also be considered the "path of least resistance". An example of this is following in your parents' footsteps. Another example could be someone's part-time job turning into a full-time career.

Addition

I found these two article which describes the work of John Holland and his theory of career choice. (not the scientific essay as requested in the comments but perhaps with more time I, or someone else, could elaborate on this topic further. It is definitely a good one)

http://www.psychologytoday.com/blog/mr-personality/201106/the-psychology-career-choices-why-personality-matters

http://www.careerkey.org/choose-a-career/hollands-theory-of-career-choice.html#.U04FZfldXh5