Going into Reverse by itself...

aha!

 

The OP is a truck driver so I'm going to go with operator error. Both my wife and I came from manual transmission cars and found ourselves more than once accidentally shifting into R instead of D when we first started driving the Prius. First gear was up and to the left on our manual just like R is on the Prius.

I'd suggest the OP consciously watch (eyeball) his shifting and see if the issue goes away.

 

I've never touched a shifter less than on the Prius. Apart from reversing outa the garage or parallel parking, I put it in Drive and there it sits, 'till I shut the car off, and it puts itself in Park.

Once in a blue moon I put it in Brake, descending a ski hill, that's it.

 

happened to me once back when the car was new. I don't know if maybe it's cause I slightly pushed it up by accident, luckily there were no cars behind.

Has never happened since.