I think its the amount of 3pm's. I havent played more than two seasons, but on both instances my player who scored in my team most 3d'ters (different each year, Kapono on one and DerMarr Johnson on another) and were not far from the top of the league with that made it into the contest.
But yeah, not sure really. Dunk Contest and 3PT one are really just an annoyance in dynasty imho, yes good that they are there but it isnt hyped enough (really isnt). You cant even get the option to see the contest where you arent participating, and dunk one is really more super-human contest than a human one
So yeah, it depends on luck alot. Live isnt a real-life dynasty simulation yet, where media and popularity plays a big role. Like 98 all stars game that had Kobe as a starter though he was a sub in regular season. Or like Yao getting the most votes because of hi-rate of asian voters. Its a popularity contest, both of them really, and dunk contest shouldnt be random but more like sending someone to the contest. Perhaps next year. Media news, between player grudges (but not very specific, just saying that this player has got problems with that one, I dont want this Total Club Manager shiet when one player scratches another players car and they have an arguement because of that).
So yeah, no worries. Youll make the 3pt contest someday and when its over its over and apparently wasnt anything special in the first place