Thinking about that one comedy anime gag that always seemed to show up in the 2000s, I never knew what that was called

WARNING! TV TROPES LINK AHEAD!
The trope is Face Fault

I think it's more specifically that the character was building up to something, putting listeners on the edge of their seats / making them lean close in anticipation. Then the payoff fails and all the tension goes out of the pose, making them topple.

I have something… (leans forward)
Something really great… (leans more)
That will revolutionize the world… (really leaning now)
The Cybertruck! (falls over)