The problem was due to 1 ATC workstation computer.
When it started up after the DDC, it had problems communicating with other computers in the system.
When it came to split out the sectors from this workstation, it couldn't copy it's Flight Data to the new workstation. So when ATC tried to use the new workstation, it received it first flight data update and goes "I don't know about this flight, I'd better restart".
Only the sectors on that sectors on the 1 problem workstation were affected, but nobody could tell what was the problem from the Monitor System. So they didn't split the other sectors.
This is a problem which has been around for a while, but has been deemed my management to be not urgent, and isn't due to be fixed for another year.