The cat (or another exhaust part) was probably screaming hot from the continued near-full-throttle lock and wrong mixture. That's your likely ignition source. If the QD that came apart was behind the bolted-on cover, then I'd say tampering was unlikely.How long had it been since the QDs had been separated? I've seen them loose after a long time.
The red thing by the helmet lock - look at another Adventure from that angle and see what is there. Could it be a reflection or possibly a glowing exhaust part that you can see through the subframe?