It could be related to the changes that allow zombies to push other zombies up. It may also let them push others to the side.
Going back to A21 there is an episode by Neebs Gaming Cul De Sacked,
that if combined with the new expandable horizontal movment. Would make
a horde attack over a larger 2d surface width and height. Similar to World War
Z the movie.
Presently they still kind of work in a line, and sometimes hop on
each others shoulders. If invisible colliders were added to the back of zombie
models, these would lay horizontally to the ground, be full pass through for
all except the foot collider be it biped, crawler, or
dog, and at different
body sections on the zombies. It could change them into a zombie body ladder.
It would be only active while zombie is still alive, functional, I don't know
the nomenclature. Because if it was still active when dead, it would create a
bunch of invisible gates, ruining tracking and directional movement.
So that instead of a jump they would progressively climb to different levels
attacking a greater and higher surface area, and create a swarm, using their already
available climbing mechanic, and swarm block damage bonus.
The reason; if it's possible to use random body segments is for variable height
attack vs just a shoulder jump which would look a little odd. Basically it
would form a body Zombie body Pyramid of Destruction.
The other visual effect could be, more of a writhing swarm. Meaning kill one on bottom
affect those above also. Have a zombie turn and those above fall to the ground. Picture
a pile of maggots or locusts.