I don't know what caused it as I wasn't in today, but it sounds like a poisoned flight plan issue. We've had them before and I know a lot of work was done to remove the logic that can cause it, but they can be VERY difficult to diagnose on the spot.
Due to some of the data flows and synchronisation issues there is a very limited amount of time to restore full electronic co-ordination before we are forced in to a manual mode of operation.
Once this time has been passed then unfortunately the traffic volumes have to be driven down to the point where manual and electronic systems can be synchronised again.
I'm guessing the delay in recovery was partly due to the fact they wanted some assurance that when they lifted the restrictions the thing wouldn't fail again...
..as I say - I've not been there today so this is a bit of guess work on my part