This seemed to be fixed in a previous update but now seems broken again
http://feedback.arma3.com/view.php?id=11818
This is now happening at random so providing a repro is proving difficult but it is happening.
It seems an easy way to see it is drive a vehicle at the careless units and they all start throwing grenade
They will throw grenades AT soldiers but this is less common.
- UPDATE -----------------
Actually this really doesn't matter that much if I put them on never fire it fixes the issue.
I forgot that that was the purpose of the previous fix.
{F21366}