May not have been the commission, but some kind of VA incentive on offer which are regularly offered to travel agents.
I do not know why Q mgmt do it BUT (see my postings on extra 'taxes' loaded on to Q international flights to same destinations as Jetstar leaving within 30 minutes of each other where Q has everything outside the fare - commissions paid on fare not & taxes - so TA gets bigger commission selling non-Q flights - Q cutting off Q International nose for some reason) they do deliberately push traffic from Q to Jetstar and Virgin by deliberately 'managing' the cost of trips.
They even have stated (Feb 2012 media release on their web site) that fuel surcharges were increasing on every Q fligth but only some Jetstar flights. Even better the IATA data shows that the Jetstar planes on several routes (Syd-Hawaii) USE MORE FUEL per passenger than the Q plane on that route. Commission greater on Jetstar flight despite total cost being nearly $1,000 less. Seems Q mgmt want to really kill off Q International.
So for the TA there is a clear preference on nearly all routes for an anything but Q flight when they look at the commission they will earn (regardless of any other incentives).
If your parents had paid using a credit card then perhaps they have grounds for asking for it to be cancelled by the CC provider (only if alternative Q seats are still available though).