The exhaust has nothing to do with it.
The transmission, however, does.
If the rear wheel is turning faster than a slow crawl while th engine is running in neutral, particularly if it's already up to operating temperature, and can't be stopped by hand, this indicates one of the gears that is free to to rotate on it's shaft may be starting to bind in some way, and somehow it's firm enough to transmit power sufficiently to drive the bike forward.
Have the engine cases ever been opened up? Particularly if there has been trans work, it sounds like one of the gear bushings may have been installed with the oil holes not registered to the feeds on the shafts, starving the plain bearing and galling / seizing on the shaft. I have also seen this happen with pressure lubricated transmissions (which the 919 has) from coasting down a long mountain road with the engine off. Pressure lubricated plain bearings spinning rapidly minus oil pressure = seized bearing.
How long has it been since you changed Oil? You'll get nicer clutch and transmission action with clean oil if its due to be changed. Was this yesterday, when it was so hot out?
Change the oil and run a strong magnet through it. If it has a few minor metal flakes in it you're fine, but if it's hairy something is grinding itself to uselessness, and must be addressed.
Support the bike with the rear wheel off the ground. Spin the rear wheel in the normal direction while feeling for drag. If there is a trans problem the lower run of the chain will get tight and stay that way as long as it's turning.
I sincerely hope it's something else, anything
else, especially if that something else doesn't require a bottom end teardown. My extensive experience with transmissions tells me this is a possibility.