If it has a hot shift PTO with it’s own clutch it can be engaged without stopping the transmission input shaft. It’s possible that the clutch is failing in the PTO but I’d still lean towards driver error.
What an idiot, and you can't blame this one on drugs. I thought it was pretty funny, good thing the sign, and not the bridge. Once at one of my many dump truck jobs, the boss calls on the radio, "I just found a PTO power shaft laying in the road". Everybody pulled to the side and hopped out to make sure it wasn't theirs.