It's probably a flow on effect from the QF9 inflight shutdown and diversion to New Delhi. A replacement aircraft (+ 5th pod engine) was flown to Delhi and then loaded with pax continued the QF9 service to London. The aircraft in Delhi would have had its engine changed and flown out to SYD (probably with failed engine in 5th pod position).
Qantas currently does not have any spare capacity on most fleets and the B744 is in an extreme position. When one aircraft is out of action for 24+ hours like the QF9 aircraft the flow-on effects can last for a week with flights delayed waiting for an aircraft to become available. Often, a service will be operated by an aircraft with the wrong configuration (ie a Pacific configured aircraft will be sent on the Kangaroo route which means significantly less seats being available). Everyone is hoping that when the A380 arrives that there will be a small capacity excess in the lohg haul fleets to allow these disruptions to be handled more effectively.
I don't know anything about the extended transit in BKK.