When they are nursing, they need to drink a lot of water to produce the required amount of milk. Add to that the heat, and they will need to drink even more. I am sure they are also eating when they go to find water for a drink.
Another factor may be the coyotes and other predators don't seem to like the heat either. The heat of the day may be a good time for the deer to leave their young in search of water and food.