I guess the would be especially problematic on older rims, which sometimes have a straight vertical profile (with no lip to hook the tyre bead into). I bought some inch and a quarter panaracer paselas for my 1960s ten speed a while back, which need to be inflated to 95psi (they have quite flimsy side walls), and they popped right out of the rims!
Not had this problem with the modern rim profile though, running Schwalbe Marathonds around the same pressure (700 x 32C).
But yes, I would definitely worry if the tyre seems to be trying to push its way out of the rim!