1990 Chrysler New Yorker Belt comes off in Rain

This just recently happened to me. First occurance was when I was in a parking lot and went through a water puddle around 5-10 mph, I noticed all my guage lights come on. I turned my car off and started it up and the lights went off. And everything was fine.
Second occurrance was when I was driving home and was caught in a downpour and my lights guage lights came on and then the car started to heat up and the power steering was hard. I pulled off the road and opened the hood to see the belt has came off. I had a mechanic come down and put it back on for me.

I have had this car for about 6 years and have been through alot worse rain than that and never had the belt come off.

About 4 years ago the tensioner was replaced. Belt is in good condition according to the mechanic.

I have heard the tensioner could have went bad again and I have also heard that a pully might be bad.

Any suggestions?