What causes pieces of this donkeys tail to do this?

I would guess that stem died off at the base, or was disconnected from the soil somehow, and is just sort of languishing. Follow it up and back to see where the injury occurred. Remove that stem. Inspect the whole plant for bugs, especially the area near the top of stems. You can sometimes get a good view of hidden parts by carefully lifting individual stems out and away from the center.

Ok thank you! U r always so helpful! U 🪨!🤘