To much build up pressure in coolant system!

What did the temperature gauge show during all of this? Sounds like you overheated.

Cap failure could cause the overpressure, but it did the same thing with two different caps. Now, both caps could be bad or maybe the replacement was the wrong type, but that's a fairly big coincidence. The thermostat wouldn't have much to do with it, it either opens or doesn't. If there's hot water in the radiator then the thermostat is at least partially open. Same for the pump, it's would be impossible for it to create an overpressure by itself.

Did you run the system open (cap off) for a bit after you added coolant? The hot air in the overflow could have been at least partially because you didn't get all the air out of the system after you added coolant. But even then you shouldn't generate that kind of pressure. Did the new overflow container have a vent or way to drain if it filled up completely?