Near miss - Brake or Bearings going bad?

Can someone help me find out what is wrong with my bike (Giant TCR2 / Ultegra / Mavic Aksium wheels with about 4K miles), I’ve tried a local bike shop and they were not very helpful.

Mile 38 of a 50 mile ride going downhill at 38mph (it’s a big hill) and my rear wheel almost locked-up after a small bump in the road. Sounded like the brakes were being deployed but they were not. In addition, I could not move my cranks (forward or back).

I’ve checked the brakes and they seem fine and removed the wheel to inspect the hub / bearings. They all seem fine.

What happened when the bearings fail?
Could this be a problem with the free wheel?
Thanks,
pt

Maybe something lodged between the wheel and the frame, causing the wheel to temporarily lock up?