I had the EXACT same problem develop on my prior bike (Kawasaki Vulcan 1600 Classic with fairing)...high speed wobble, as well as wobble on deceleration. Bike ran fine with no problems until about 10K miles, then on a trip starting getting a very noticeable (and somewhat scary) "wobble" when I hit about 70 mph or when I was slowing the bike down from cruising speed.
Checked tires, shocks, removed fairing, re-installed fairing, etc. Turned out it was the front end "knuckle" nut - or triple tree nut that was loose. Tightened it up, problem went away. I found this is fairly common after doing some research...some bikes come from factory with nuts not completely tightened, and tend to loosen up after putting a few thousand miles on the bike.
Not sure how the Triumph knuckle tightens, but I had a tech do mine since there was some dis-assembly required to get to the nut. Hope this helps...
I'd put money on this being the problem