Are you friends with your neighbor? Would you get arrested if you went over there? Is it a LOT of water? Can we get pictures? I love pictures... Do you see a hose in the pool? Maybe they left it on... Is the pool old? Maybe the wall around the skimmer finally gave out... It happens.
If it's overflowing the skimmer basket on the side of the pool and it's not raining, I'm betting they have an autofill that's stuck open and they're not going to be happy when they come back. Either that or they left a hose on and they're REALLY not going to be happy.