Actually you can perform the throw on command. It isn't as random as one thinks. I essence, you have to "charge" the throw by defeating X amount of enemies. Depending on the character will dictate the amount you have to defeat. This is much like the TMNT 1989 arcade game. Also, you have to stand at the right distance to pull off the throw as well.
From what I've seen, Nightcrawler has the shortest "charge" time, with only one enemy to defeat. Just about everyone else needs to defeat at least two or at the very least damage a second on severely. Colossus is the only one that I've seen that needs to defeat two before it activates.
