The rear cargo hold, (usually known as the bulk cargo hold), is separated from the rear belly hold by a removeable cargo net.
The bulk cargo hold is normally used for lose bags, small freight and unusually shaped items like surf boards etc.
I am only guessing but it is possible the flight departed SIN with a u/s door so they would wish to keep it closed until there was time for it to be fixed. If required the bulk cargo space would be accessed via the rear belly hold.