Impeller swelling and seizing?

[Reef Pilot]

What causes pump impellers to swell and then causing seizing and hard start-ups? I have had that happen to a couple pumps (different brands) so far, and then have had others running for many years without a problem. Is this just a defective impeller or some other cause, like not cleaning enough? These pumps are run underwater so can't be because of overheating. Or is it just bad luck?