Kinda simple. Don't have any links, but I am sure there is plenty to be found on medline, or pubmed if your not a subscriber or holding an account.
Prolactin is the horomone that triggers breast milk/active mammary glands in pregnant and post pregnancy women. That is how it induces gyno, it says "make milk bitch!".
Rather simplistic, and there is much more to it, but thats the jist I guess.
-FC