Ah. I was wondering about those. First time I hit one, I was pretty angry about losing my hard-earned fastfire. Makes a bit more sense now, I suppose, since the level has so much speed destruct scenery. But yeah, my brother made a large level with a bunch of trigger crates that wouldn't show up. Same issue, except he didn't fix it.
Though, how do you kill the player without losing a life, then? Was that in Fortress of Decay? I suppose I wasn't looking at the life counter.