The A380 must have had a mechanical that pushed them beyond the curfew. Sometimes even if they manage to get an exemption the crew may time out since the JNB/SIN flights are pretty long. Then the only alternative is to restart about 11-12 hours later. One of the reasons not to push it to 4.45 next morning is the pax. They certainly cannot be expected to sit for 7-8 h in the airport or return quickly enough if you send them into a hotel.