That sucks for sure.
Usually, if there is a wasted bearing and/or a darkened/blackened journal, that's what caused the failure.
Why that happened in your case.......that's what you need to try and figure out.

My friends "good" stocker motor ate a rod bearing on the dyno last fall, and that's a motor that has been run for years, and been on the dyno many many times.
The power was down just a tick and it wasn't repeating as good as it usually does, but that thing uses a Q-jet, and those can be fussy. We did a couple of carb swaps, seemed to be making some progress, and actually got to a point where it made the most it ever had.
We took a break and went for lunch. After we came back and resumed testing the power was down about 15-20hp. The first pull after the motor has sat for a while is usually down a bit but not that much.
Oil pressure seemed a little low after the pull. Loaded the motor to make another pull, TQ looked like it was way down....... And I just shut it off. I knew something had gone away.
Pulled the filter and took it apart(system 1), saw some filings.....we were done.

I don't recall which rod bearing failed, but all the rest of the bearings looked perfect.