It takes Pratt a minute before that really sinks in, "You jumped off the cliff?"

Maybe there's other answers here, maybe Carlisle shoved it, maybe it fell, maybe it didn't see the cliff and walked off it. But the most likely answer is that Carlisle, out of options, jumped into the water from a cliff and this thing tried to follow.

"Jesus. I'm glad you got away." Though it doesn't appear as if he escaped unscathed. "Were you injured?"