I see a lot of these pumps over here when I have to help fix things ( on an island you get sought out when things fail )
Full synchronous motor, and provided the coil is in tact (easily rewound anyway), they will always go back into service.
The main thing that stops them is simple blockage ( covered already here), and fine materials getting behind the front seal, and down into the magnet area. This will cause a slight stiffness.
Being synchronous without a start up system, the magnet oscillates between clock and anti clockwise at start up,. The "clutch" in the impeller makes it frictionless for most of a turn, and then grips. By then the magnet should have start ed to follow a field, and have a direction, and the impeller now is driven by the rotor via the slip clutch in the head of the impeller. If it does not on the first pulse, it "bounces" off both the impellers inertia, and impeller vanes against the water via the clutch, and heads in the opposite direction, and tries harder to catch the moving field. It may do this a few times, bouncing back and forth from the end points and the fields.
The only thing that can stop this, is if the impeller clutch is jammed or broken, so not giving it a frictionless first half turn, or if the fine muds clogged in the rotor area are making it less than completely free turning. In that case, the rotor can't ever catch up to the driving field, as the rotor inertia appears too high, and as it starts off in one direction, the ac wave will change before it can get up to sync, or bounce at the "end", so it has to reverse on it's own and try again. Without friction, and a good clutch, it is fail safe, but if anything slows the first rotation/s / oscillations, it will not be able to catch up in time before the next cycle. and just oscillate.
I like this style of motor as they are mostly able to be pressed back into service without difficulty.
..................oztules