It can sometimes be associated with feeding aggression, but we don't see enough of this behavior to really become authorities on it. If a mare has space to move around, she can get away without trampling her foal, and the foal is far less likely to be injured or killed. If they're in the open, the owner can observe and figure out what's truly going on between the two, whether it's overprotectiveness, fear, or true aggression. Even a sick or weak mare will usually gather all her strength to retrieve the foal and bond with him. If she doesn't respond right away, the foal will keep trying for up to about three to five days, attempting to get her to recognize and protect him.
Overprotective mares can benefit from more space as well.
A bucket of grain can be a powerful persuader for such a mare, but you'll need to remain cautious around her until you're sure she's truly relaxed and accepts you. That said, there is such a thing as too much intervention. Close confinement and too much human interference seems to increase the risk. It's certainly important not to go overboard in terms of intervention. One case where intervention is necessary is when the mare won't allow her baby to nurse.