I often see discussions on tank slappers/wobble or weave. Usually riders saying that it doesn't/does happen to them under same conditions on their bike.
Dunlop looked at this and made a short film which is really great..it is some years old now, so sorry about the quality..
https://www.youtube.com/watch?v=z3OQTU-kE2s
Easy to find by searching for 'Dunlop wobble weave video' incase my link goes down..
The video explains much; basically the biggest culprit seems to be a squared off rear tyre, which has been noted as the problem in posts before..
Heavy top boxes and panniers also make things worse, whilst pillions help to reduce the chance of encountering the effect.
Onset speed is controlled by rider mass, all other things being equal. Being a pie enthusiast is useful in this case! Higher rider mass gives a higher onset speed.
The cure? Lying down onto the tank, asap!
Hope you enjoy the video as much as I did..