Yeah it's only ever happened once to me, and that was the weekend before last when the instance servers were lagging hardcore.
From what I can infer, it's basically:
1. When the player is spamming buttons very hard, and
2. They get lag / pl / server hiccups.
I wonder if it's a similar issue to the DOT timer ticking down as 4..3..2..3..2..1..2..1..0..2..1..0