Scary moment!

I was just coming home and as I was approaching a roundabout at around 50mph I began to slow down and the rear drivers side wheel locked up completely!!!

I managed to keep it pretty straight considering and luckily there were no other cars nearby to worry about. Once I'd stopped I got out and had a look but couldn't see anything obviously wrong (wheel on it's side or anything) but just a mahoosive black tyre mark on the road about 100 yards long. I got back in and tried to move it forwards but could feel the wheel still locked. Thought I better move it to the very edge of the road so started dragging it and it freed off. As I was only a mile from home I limped back carefully at 25 mph (it drove fine) and it is now in my garage ready for inspection.

All I can think is that something has broken in the rear drum, jiggled its way about, and then jammed the drum and caused the lockup.