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 stuff even in real life, and bunching up happens all the time on the battlefield... Units know its wrong and possibly deadly, but it still happens.