It's the bike, most likely the fork mounted windshield, maybe bags too. I chased all the usual stuff, it's just unstable at high speeds, kind of a slow back and forth rotation along it's longitudinal axis, right? I think the wind builds up on the windshield, and spills left and right. That and maybe the tire cupping, new tires are better for 1000-1500 miles. Starts anywhere between 120 and 140 mph for me, never really gets worse, but scary the first few times.
I'm guessing Triumph knew, hence the low speed limiter and lower rated tires on the Touring vs Standard/Roadster, doesn't seem to affect Non- Touring bikes.