Yeah, it's semi-funny because there was clearly no human intervention causing the cat to go into the pool, although I didn't laugh. It's possible that the offender possibly did it to initiate play and this was an unintended consequence, given how it ran after it with its tail up afterwards. However, it really does look quite deliberate to cause the other cat to jump into the pool, to me. Why be mean to it?
I've also seen a video of one cat being really mean to another and there's no other way to look at it, so this could have been premeditated too. In that other video, one cat was just sitting upright, contented and closing its eyes. Behind it were some kitchen cupboards, where a second cat slealthily emerges, then wacks the first cat hard in the face right over its eyes, scaring the bejesus out of it, causing it to jump and run away and possibly injured it. Why the hell did it do it? Quite the douchebag thing to do.