1986 Volkswagen Cabriolet hydrolock causes

My water pump blew so I replaced it and the timing belt and started my car back up and started adding water as the fill tank drained into the radiator. The car ran prfectly and smoothly for 15 minuts till it just died and wouldnt start again. I checked the oil and found it was mostly water, I had checked the ol befor I started adding water and it was pure clean oil in just the right amount, I left the car overnight while getting a truck to tow it with and in the morning the car started easly and drove onto the tow dolly. I know it must have hydrolocked but I need to know what could have caused it in this situation. The car will still start run and drive but I dont want to have it hydrolock again and I dont want to tear apart the engin if it isnt necicary.