If the only thing changed was the front tire, there's your answer.
What brand and model tire was replaced and what brand and model was installed?
The ST1100 decel hands-off wobble issue has been around since day one. It's not unique to the ST. IMO it's the result of designing steering geometry on a heavy sport tourer that's a compromise between sport handling and touring stability. Tires seem to be the most common suspension component associated with decel wobble onset, but others have been attributed to it also, like steering bearings.
If you do experience it, just don't take both hands off the bars when decelerating. Often, only a single finger on one bar stops the oscillation.
John