Any idea why the NFL stops the clock on a sack?

[Texas Aggie]

What is it about the sack that makes them want to stop the clock when they don't really stop it otherwise, except incompletes and OOBs? Today I saw a sack with the clock stopped and I timed it -- 6 seconds later, the R started it again. Makes no sense.

My first year, I was working a frosh game as R (NCAA rules). Real good game that went down to the wire. Late in the game, I stopped the clock on a sack. I quickly realized my mistake and wound it prior to the ready.