Replaced thermostat and refilled.drove let it cool checked radiator, it was low. Topped off, drove, cooled, checked, low coolant again. Repeated. Eventually realized expansion tank it about to overflow. What could cause that?

Answer: Sounds like the coolant is being pushed out into the overflow tank. That can be caused by still having large air pockets in the cooling system. Air will expand when it gets hot. Either that or the engine is overheating. Back to the air, a blown headgasket can cause air pockets and overheating.