He may have been standing, and his knees gave out. It's worth considering alternative interpretations.

Or maybe he was in a dunk tank and he was so bowled over that he swung his arm out triggering the latch but someone forgot to fill it with water.

Another likely scenario!

Perhaps a bit less likely, but we'll allow it.

Yes! I literally fell out of the chair, which made it even more hilarious.

Yep, must have been hilarious. Thanks.