Well, as I expected, that didn’t take long:
Engine pressure anomaly traced to turbopump valve. Replacing on engine 5 and verifying no common mode. #DragonLaunch
So it’s a good thing it shut down — it could have resulted in a catastrophic engine failure in flight, something that SpaceX has theoretically designed for (they claim to have engine-out capability from liftoff, and sufficient inter-engine blast shielding to prevent fratricide), but probably doesn’t want to test, at least this early in the program. Also, they had no performance margin on this mission, so it probably would have meant an abort to a low orbit, and perhaps an inability to get to the ISS.
[Update a while later]
Here’s the story over at Popular Mechanics.