Because he's an opposing player who's been skating circles around you all night (though I didn't think DaCosta did much last night), and you have a chance to trip him up, so you stick the leg out. Desperation? I will agree that the idea that he stuck his leg out with the intention of smashing his knee into DaCosta's in some modern version of the "smash two eggs together and see which breaks first" game some of us played with easter eggs; that's just asking to wreck your own knee. Sticking the leg out simply to interfere, though? Totally plausible. And again, if that's what happened, Phil should be out for a game.
I'd say "there was no penalty called", but those refs were just as likely to call DaCosta for boarding as DeSimone for tripping (i.e. they were bad).
Given MC's speed, the small ice might help UNH tonight.