Why are dealers always knocking over stacks of chips?

[DimensionPresident]

I've noticed that dealers are constantly knocking the stacks of chips over. I can understand if the player sloppily throws the chips in, but even sometimes when chips are stacked, and the guy wins the pot, the guy will be going for the stack, and it's like the dealer hurries up and knocks down the stack so the guy has to re-stack his chips.

Is there any real reason for dealers to do this? I think I've noticed it enough times that I've ruled out coinsidence.