It doesn't happen that often. Possible to be highly common in commercial situations, but shouldn't happen much in pasture.
The piglets huddled against mom for the night that I saw late last night. Mom goes out to potty and comes back, easily come of been too hard to see where a piglet wandered off to and got in the way. If she laid on it with most of herbweight, even a few seconds could of caused too much internal damage for the poor thing to make it. He was bruised up on one half side, but not flattened, and he was tucked into some hay off to the side like he tried to warm up, but never could. So mum must of realized it, but it was too late.
I know a lot of pig farmers would cull for that, but pigs can see well in daylight, let alone at night.