It seems like the AI won't reverse a IFV-6C properly at times, it just wiggles the vehicle while reversing at a very slow pace. I'm unable to reproduce this at all times, but I'd say out of 8/10 times I'll manage to make it reverse like this. It might take a one of two attempts; reversing, forward, and then revering again.
Beta version used: 0.75.10824 (existed since vehicle was added)
EDIT
It seems this also affects the BTR. I'm changing the title to include a more general 'tracked vehicles'.
(4.) If the problem didn't occur, try command the AI to move forward a bit and then ask the AI to reverse again.
Not sure what causes this, but I would guess it's something about the vehicle model itself, as there's no other vehicle that I'm able to reproduce this on and I'm only able to do this with an AI - and not a player - which is why I'm placing this under the 'AI issues' category.
I'm interested if anyone else can reproduce this since I only can reproduce this sometimes.
I cannot reproduce it (i.e. don't have exact steps needed for it) but can surely say that I encounter with this problem myself on a regular basis. Also I think your guess that it's something about the vehicle model itself is correct - sometimes (not always) when AI won't reverse normally I go to driver seat and experience the same thing - it twitches backwards for a moment and then stops despite I'm pressing "S" as hard as I can =)).
I've just played a mission with Slammer tank and the AI driver was unable to reverse, moving slowly forward instead. Switching to driver's seat cured the problem, as tank was fuly able to reverse under manual control.
Fully reproducable with all tracked vehicles, on road and on the airport runway.
Tested with today's DEV build [114862].