Normally, pointing somewhere gives a context option like "Move" or "Attack". Directly pointing at the controlled vehicle gives the context option "Disembark".
However, AI often gets the "Disembark" order as preferred option where a "Move" order would be sensible. This makes AI disembark in the middle of combat which is senseless.
Reproduction is hard. When just putting AI in quads and moving them around, everything works normally. It usually happens during firefights when giving a move order to flank an enemy or similar.
One possible explanation might be low framerate and the cursor swipes across the area of the vehicle. I pay attention to where I click but not to where the cursor has been before it gets to where I want to click, so I'm speculating.
I recommend removing Disembark as preferred context option (second line, where the most sensible context option is highlighted). It is still a quick option when scrolling up (for beginners who don't know the menus) and can be accessed any time by pressing 4 and then 1. I cannot think of a scenario where pointing at a car to make AI disembark would make sense.
Especially since "Move" and "Attack" are by far THE most important commands in a tank battle when commanding via the map.
Also, this has been around since Arma 2 and hasn't made sense back then either.
I've just found out via testing with quads that the Disembark option simply stays selected when hovering over the vehicle and then moving the cursor away. So just touching the vehicle when moving the mouse is sufficient to make the player command "Disembark" when clicking anywhere else. Tested in first person. Reproduction unreliable. Happens sometimes but not always.
Edit: Further testing, I found a way that seems to reliably work. 3 AI, 2 in an MBT (Kuma) as driver and gunner, 1 in an APC (Gorgon) as driver, me as commander in the APC. Selecting the gunner AI of the tank I don't control gives me "Disembark" as default option regardless of cursor position.