Maybe I am naive but I was amazed that Qantas have crew switch planes on a shift.
Any medium sized transport operator - surface or air - may do this, particularly if they have a complex network. There will be numerous reasons including hours of duty and perhaps even 'variety', although obviously cost effectiveness is a major consideration as are flight plans for the next day, as loadings in any passenger transport business vary from day to day. QF like most airlines may rarely operate an identical number of flights from day to day across its QFd network in totality. Staff can go off sick, affecting availability.
One further example of a crew switch may be if a freeway, tollway or rail line leading to the airport was closed suddenly (or there were major delays) and hence one crew complement was delayed in signing on to start a shift. Another 'purloined' crew may need to operate a flight that was not in their roster. And if that inbound flight is itself late (but not as delayed as the crew stuck in cars or on an airport train), that means a late departure such as you experienced.
Remember also that planes of the same model and configuration can themselves sometimes be substituted for one another to try to maximise on time running, so it's not just the crews.
On a different note, Wednesday 15 March has been extremely wet along the north coast of NSW (but not inland from what I could see) with CFS for instance receiving more than 100 millimetres of rainfall between 0900 hours and mid aftenoon. QF2108, the 1130 jhours SYD - CFS departed at 1220; arrival was at 1336, 51 minutes behind schedule.
QF2006 from SYD to TWM, the 1525 hours, departed at 1630; arrival is predicted at 1749, 69 minutes tardy.
The 1700 QF147 SYD - AKL is forecast to depart 45 minutes late.