No idea, we may well never know, but the probability is (based on stats over the years) it was a medical problem...
To be frank I think, albeit unknowingly, there's almost literally an element of ambulance chasing going on with this fixation on the 7700 squawks.
Most technical problems (similar to what likely happened to the AA flight last night) are v minor, get resolved in slow time and the aircraft retains it's original squawk.
There are some circumstances a pilot would go straight to 7700 without prompting by ATC (e.g. decompression) but otherwise it often only gets used if ATC ask for it, and that’s often done because it gives them more visibility (on radar).