Marcus Smart just wanted to help his team. As the closest Celtics' defender to the paint, Smart naturally decided to slide over to fill the gap once he saw a play begin to develop. Unfortunately, this play involved DeAndre Jordan hurdling toward the basket at full speed with an alley-oop in progress. Smart might've been lucky to come out relatively unscathed:
You could mention that Smart, a 6'4 point guard, probably shouldn't have been stuck in that position, but let's be fair: pretty much nobody stands a chance once you've let Jordan, who is listed at 6'11 and 265 pounds, build up momentum like that. It's just simple physics, really.