A cock treader is someone who steals girls from another guy, usually (but not always) just to piss the other guy off.
"Dom picked up this hot girl in the club, and was convinced that he staying at her place tonight, but I totally trod on his cock and now Dom is going home alone! I didn't even like that skank bitch!"
Guy #1 "Dude I finally had the balls to ask her out for a drink, and she said yes!"
Guy #2 "Damn shes hot, i might just have to have her for myself!"
Guy #1 "Man you're such a cock treader!"