This comes from Mitch.

"I’ve been working on making the zombies clump up less, spread out when pursuing prey more, and behaving better when they lose the ability to get to their target. They’re also attacking on the run, so players cannot simply back away from them and be safe. The video show several different groups of zombies, each with a different speed profile.

In the active pursuit, several faster zombies catch up with me as I’m running backwards while firing (to attract as many of them as I can). Eventually, they catch me, grapple me, and surround me. I am sitting on a rooftop that the zombies cannot get to, due to several closed doors. They circle my location, keeping an eye on me as they pass. This lets me know that I’m still registering on their sensory systems (and am still on their list of targets). I’ve been working on these pathing enhancements for a few weeks, and while I’m still enhancing the systems, we expect them to go into testing shortly."

Source: Reddit