The two forwards were jostling for the puck as they exited the Montreal zone, and St. Louis seemed to get his stick tangled up a bit with Desharnais as he went for the steal.
Desharnais was going to get the call against St. Louis anyway, but he decided to put a little extra into his performance:
Bravo! I've seen plenty of dives (see: Kings games) before but I've never quite seen someone try to embellish a call after they've drifted five feet away from the play. Desharnais was deservedly penalized for embellishment, and St. Louis was called for hooking.
We now know why Desharnais actually fell down.