I think QF stands to gain a lot more by flying to DFW instead of SFO than UA would gain by flying to IAH instead of SFO.
UA has a hub in SFO; AA really has...nothing.
There's also arguably a lot more O&D from SFO to SYD than IAH, and I'm sure a bulk of SF-based frequent flyers are in bed with UA, not AA (and thus, QF).
The QF move to DFW makes sense. The UA one to IAH...not so much.