Pump not restarting?

[Reef Pilot]

What causes pumps to not restart properly? I have a Sedra KSP-20000 (2000 gph) return pump that is about 1.5 years old. It has run flawlessly until recently. I shut it down for water changes and noticed the last few times that it would chatter a bit before starting up. Then yesterday it would not start up at all, even with repeated unplugging. I removed the outlet hose, and was able to rotate the impeller a bit. I reinstalled the hose, and it started up again (was very happy about that).

But now I am concerned. It runs a lot of my stuff (with T's) including pushing water up a level to my main floor tank. If it quit working, or failed to restart after a power outage, I would really be hooped.

Does it need cleaning? Has the impeller magnet swelled (happened to a Koralia I had)? Worn impeller parts? I haven't taken it apart yet, but may do so after the next water change.