Originally Posted by
banerjek
Not seated properly.
When you mount a tire, the best way to prevent this is to inflate the tire to about 20 lbs or so, work the tire into the rim to make sure it's seated, and then inflate the rest of the way. Yes, it is possible to go as far as you did before it popped off.
I do a version of this when I mount tires. I inflate to 30PSI, then deflate totally and squeeze the tire beads together all around while checking to make sure no part of the tube is visible or trapped anywhere. I then inlfate to full pressure, and look all around to make sure the tire is well seated.