Scarf Metagross vs. Non-Scarf Hydreigon in TR does make sense. The opponent would have to be REALLY stupid, granted. But a Metagross on a TR team as a "backup" for when TR is gone is possible, I suppose. And he just got forced in prior to TR turns running out.
Really, without knowing any other details of the battle it's impossible to tell. It could be that Gross got the Scarf Tricked onto him, which would make a TR team with a ScarfMetagross hurt my brain less XD
Otherwise, it'd have to be Quick Claw, like Pocket said. It activating could go unnoticed in a fit of "I'MA ROAST YOU" fury.
] <MK_Ultra> that kid is asking to be molested