If you use the Rotex in rotating (coarse) mode, which you need for polishing , then the rpm is merely 660. Which is very slow, even for polishing. But the sheepskin should never fly off the Rotex no matter which mode or speed you use.
If your sheepskin flies off it might have a flaw or you didn't put it on in the proper way. The hook and loop of your pad could also be worn.