"William Sommerwerck" writes:

My guess is that it's to even out the wear.

It a function of the type of motor used, it is much smaller to make a
motor that will turn in either direction than one that always turns in
the same direction. If you look at some extremely small motors,
some have a ratchet and tiny gear that only allow them to turn in one
direction, but the ratchet and gear take up space and cost extra $$
and so Uwaves don't bother to use them.

Doubt it. 99.9999999999 percent of motors in consumer appliances and
electronics are either unidirectional or may have their direction changed
trivially by the electronics, and it doesn't cost more or take up more space
or cost more.

--
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Shades of the very first AR turntable. It used two motors, the smaller of
which was present to make sure the larger turned in the correct direction.

My InSinkErator changes direction on each cycle, but this is intentional.