Unfortunately this is due to a mechanic with the AI and this had been brought to the attention of the powers that be. I used multiple zones in the AI plan to create additional random movement per AI plan and to create a more realistic deployment. When it works, it works great, but in rare cases...all of the units are sent to a single point and the AI does not deconflict that, which causes that weirdness. I have also observed that with large zones, there is still a chance that the bunching occurs as well and the AI does not spread out the units equally inside that zone.
But, units do weird