Staffing on a nightshift is somewhat less than that during the day. Should someone "go sick", then you are relying on getting someone in on overtime (known as an AAVA - Atco Additional Volantary Attendance). If nobody is available or indeed does not want to do overtime, then this creates a staff shortage and thus flow is put on to protect the controllers as you can't split sectors etc.
I'm afraid the answer to your question is "Don't know?" If the same situation arises and someone is willing to do overtime, then there is no staff shortage and so no flow. Likewise, if a body cannot be found, then the situation will arise again.
Not sure where the shortage was i.e. in the Tower or at T.C. where the approach function is performed. Interstingly, the airlines prefer that delays occur in the evening or at night (if they do have to occur), as this has the least impact on their schedules.