I agree with wolfchild.
The most common cause of the blow off that you described is pinching the innertube under the tire bead while installing the tire. When you do that it may hold air for a while but eventually the air pressure in the inner tube will find a way to push the tire bead past the rim.
That can happen with hook beaded rims too, just not as often. Back in the days when I rode with only a CO2 cart in my puncture repair kit I learned to be very careful to make sure the inner tube was enclosed in the tire all the way around on both sides before reinflating.
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