this might probably be "rage mode" which is a factor to somewhat add rng to a otherwise too predictable - and more importantly too congeneric - zombie behaviour.
Guess how stupid it would look if every zombie would show the exact same behaviour based on the same "formula" all the time.
This would make things too predictable and therefore lame and easy to cheese.
My current horde base is integrated in the radio tower of "cell tower #1", with the main gate wide open and then a staircase up the fighting position.
So an absolutely clear and wide open path to me, but they often first bang on the walls and fences a little before actually taking the clear path to me.
Some even stop in the middle of the stairs to punch the air a few times.
Temreki supposed in one of his vids that this might be an issue with the new "simplified zombie AI mode" system, but I don't think this might be it in my case...I don't have that much going on with my settings on hordenight that this should be an issue.