@mechajutaro said:

the little girl at the beginning made him jump out the window and the crash made him conscious ? :)

Not sure I follow your reference, Wolf, though I'm guessing The Exorcist trilogy. Karras jumps out the window in the original then resurfaces with another identity in Legion. Correct me if this inference is in error.

No, man, the only reference I thought of was to Johnny No 5 (or whatever that robots name was). He was struck by lightning and became conscious.

I've never watched The Exorcist movies, but good to know the reference you mentioned. :)

The question still stands though, why was Andrew conscious in the first place ?

To misquote a line from "Futurama": "Each robot experiences the upgrade differently." An accident of manufacture, a changed/extra line of code, or the difference between how the girls treated him are among many possible causes for "individuality." In 1999, such questions were the stuff of Science Fiction, but one day soon the "what makes us human and them machines?" question will have to answered. Why would a "servile automaton" choose to be "human" is a better question. After years of "Terminators" and "self aware machines" this is a softball tale begun with D.A.T.A. of "Star Trek: The Next Generation," which avoids the Reproductive Test as the final "measure oF A man." (But then, in the movie "Demon Seed," a super computer got around that test as well.) Oh well, see you on the other side of "Proposition Infinity."

Arthur C Clarke's 'Positronic' brain is the answer. Self learning, so being placed into an environment where it is treated as 'family' allows it to acquire more sentient set awareness. We are all, after all, the product of our early learning and upbringing.
No more explanation needed.