Who knows? Could be random, could be a raffle of the post ID number as shown in the lower right hand corner of each post, could be karma?
Going back to the 15millionth post contest, only the grand prize was actually the 15mil post. There were LOTS of other winners. EVERYONE was having a LOT Of fun trying to entertain each other in that thread, and a family of friendships was born in that thread. I don't know how they picked all the other winners, but it did seem that almost all of them were the ones having the most fun with it. And the Grand Prize winner, USMCotaku, was definitely high in the Karma dept. in THAT thread. You can't do much about your post ID #, or randomness, but Karma is in your hands...

I'm guessing ECF just wants you to have fun, and entertain each other.