I found out.
In Generations 4-5, the player's voice can be recorded and this recording is played back when the move is used. The louder the recording, the more likely Chatter is to cause confusion. The chance of confusion can be 1%, 11% or 31% for a low, medium or high volume respectively.
Chatot was previously banned from online play due to the possibility of inappropriate phrases being recorded with Chatter (although the recording is always somewhat distorted).
From the DB page