Weeaboo anime thing.
Just like how they fall over whenever someone says something unexpectedly stupid (there's a really funny one from like s1 where Pikachu like jumps across the screen when Ash says something dumb) or like the whole blue lines under the eyes when they're upset or the sweat drop at the back of the head when nervous.
DBZ did all these things as well. Infact, there's an episode of DBZ where some of the characters are having a fight inside and there's an exterior shot and the house has a huge sweat drop on it's back. Kinda weird.
