I doubt the BB for 2 reasons: 1- as stated above you would be effectively tightening the BB as you pedal. 2 - For the type of slip that occurred, the spindle of the BB would have to be "seized" to engage and turn the cup so much at once. In other words, if the BB was loose, I would expect it to be gradually tightened during normal pedaling.

Curious that the hub is pretty new though. I would take it to the shop and have them check it out. This is just my best guess as to what is happening; I'm not a bike mechanic, I just play one on TV

Keep in mind, this will be one of those problems that are hard to diagnose or repeat on the workstand. You may just have to ride things until it happens again!