That's definitely when I notice it most; can't say it's the only time it occurs with confidence. I can go look at some vods to see if anything stands out.
Just going from memory, there are a couple things I've noticed when this occurs: I'm predominately using short blades, I've already interacted with the zombie (stabbed/pushed), and it's most noticeable when backpedaling or kiting.
EDIT: Happens with as few as 4-5 zombies. Hard to see what might trigger the change in grabbing behavior; i.e, whether it's the zombie being stabbed or forced backwards into another zombie via push/hit/stab. Whatever the cause is, it seems like the event switches that particular zombie's behavior/visual and they will continue to attempt "long grabs" when possible.