Just guessing here, and I'll double check when the other translation comes out, but I believe the way Mamezuku died here was because Josuke had placed soap bubbles inside the walls, so that when Rai attacked the wall the bubbles travelled up his arm into his body, and then popped out taking chunks of him with them. Obviously this was meant as a trap for the doctor, but because Calamity, Rai was the first to accidentally set it off. Another explanation is that perhaps he simply ran into the bubbles floating in the air on his way to the doctor, and Josuke had set those bubbles up to damage people who touched them. Either way, Josuke set a bubble trap and Rai accidentally triggered it due to Calamity, it wasn't like Wonder of U suddenly had control over all of Josukes bubbles.