What can cause a pool pump to run backwards and how can you fix it?

If it was misswired, it could run backwards, don't know of anything that can fail to cause that.

The only thing I can even imagine is a pump impellor that is losing blades and the broken parts are changing the flow. This would be my first in 60 years though.

The socket to the pump may be wired wrong(reverse polarity)check the plug or the wiring from the fuse box to the pump make sure positive and negative are in the right order,r if the pump has more than 1 gear you may just need t flip a small switch in the base

Reversed polarity won't do it, but swapping wires in the Motor can.

The pump impeller appears to run backwards, but it's not. It's a vacuum pump, not a pressure pump. If there's a leak at the pump skimmer lid, it can't pull a vacuum. Lesson: check the most simple aspects of vacuum loss first.

Ed