Unplanned diversion - overrunning works?

The Busway was closed during this morning's rush hour south of Cambridge Railway Station. Does anyone have any further information? I've checked the County Council, Stagecoach and Whippet Twitter feeds, Facebook and web pages, and none report any problems. The driver of one bus was clearly unaware, so it seems like a last-minute thing.

My guess is that it's "overrunning engineering works" (a disease transmitted by the neighbouring railway line, perhaps...).