OK, I'm persuaded to the brake cooling thing given that two consecutive departures did it. The odds of both having some kind of mechanical must be close to 0 - didn't see either land back, but that would have taken a while anyway being close to MTOW.
The taxi for 09R at MIA is effectively an 11,000 ft backtrack, so presumably the decision for a gear-down climb out would have to have been made before leaving the gate.
But back to bigjets' question - what was that 767 doing at ORD then ?