Your dog has found something fun
When dogs escape and discover something exciting beyond the fence (another dog to play with, a field to run around in, food), they may keep trying to get out so that they can go have fun again. Why were they trying to escape in the first place? Probably because they were bored.
Your dog has found a friend
If you have an unneutered male dog and there’s an un-spayed female in heat somewhere in the neighborhood, he will smell her and he will try to find her — and have enormous incentive to get out of the yard however possible, whether over, under, or through the fence.