I'm pretty sure this is related to the grab delay, although you probably knew that. What happens, as far as I can tell:
1. You look at them, and a moment later they are grabbed.
2. You're swining your mouse around, so you stop looking at them, but find your aim on them and look at them before they cease to be grabbed (due to the delay again)
3. They are released from the grab (due to the delay). They move for a moment, and then are grabbed one last time (since you looked at them again in step 2).
4. Because they moved, you are no longer looking at them, so the grab doesn't kick in further.
5. If you look at them after they move, they continue to move for a moment before being grabbed. However, if you are unable to keep track of them for every moment while the are moving, they will be released from grab temporarily, and will be given another chance to move away from your crosshairs. This will continue until you either succeed in looking at them for an entire delay period, or they get away.