They normally start in the corners. They will farrow in the warmest spot in cold weather. The coolest spot in hot weather. They don't like to farrow beside an older litter. They won't farrow in a hut that has had a litter already farrowed in it even if the litter has moved on.
Sows require a lot less attention farrowing on pasture.
If your lot only has one shady spot expect all the sows to farrow under it if its hot. That would be a disaster. Keep age in the lot very close. The most important thing to remember when raising pigs farrowed on pasture is that little pigs need colostrum. A sure way to fail is to allow pigs to get old enough to be running around robbing milk from young litters.