Zombies have a rage mode that can randomly occur. This overrides any setting for walk, run, sprint.
I have heard people say it is triggered when they are hit, but I've seen it regularly when they first spot you, so I don't think it is only limited to being hit. Besides, except right at the very beginning of the game, you are likely killing zombies or knocking them down after one hit, so limiting it to being hit makes it pretty pointless. Having it trigger when they see you makes more sense to me.Even when they haven't been hit? Because a lot of our wandering hordes include ferals that are considerably faster than the other zombies - and it's totally random who is doing it, but the lady zombies are the biggest offenders
<property name="MoveSpeedAggro" value="0.6, 1.45"/> <!-- slim, feral -->