Still a lot unknown about this collision. But I've seen the scenario peekay4 mentions, in real life (and no doubt others have seen similar, too.)
Visiting the tower at TJIG (small downtown airport in San Juan) - tower clears National Guard Shorts 330 to land on long final - goofer in a 172 on downwind turns base right in front of the Shorts. The tower saw what was up about the same moment the Shorts announced "We're, uhh, doing a 360 here for spacing" (with which the tower concurred), and the controller told the 172 driver to (ahem) come talk to the tower after landing. Also told the other small fry in the pattern to extend their downwind legs to give the Shorts time for its circle.