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

[APG]

Originally Posted by bisonlj

Must have been an error or there was some other reason the clock was stopped (i.e. penalty, injury, time out request). I've noticed they don't stop it if the sack is within the last 2 minutes of a half. I guess they don't want to the offensive team to benefit from a good defensive play when time is of the essence.

The exact rule as written:

Rule 4
Section 4 Stopping The Game Clock

The game clock operator shall stop the game clock (timeout) upon a signal by any official or upon his own positive knowledge:

(g) when the player who originally takes the snap is tackled behind the line of scrimmage

Exception: After the two-minute warning of a half, the game clock shall not be stopped.

A.R. 4.6 Quarterback A1 drops back to pass and is tackled behind the line.
Ruling: Stop the game clock until the ball can be respotted at succeeding spot (40-second play clock starts when the timeout signal is given).