You are asking a pretty complicated question and one you likely will never know the true answer to.

But I would guess it's simply plain luck of the draw and you were the next up for an op-up. What's your status with QF? Being in oneworld your status with QF gives you some level of op-up priority with CX. See CX's order of op-up priority in the FAQ here. That list is not officially confirmed so obviously YMMV.