Any idea of the cause of tonsil stones?

Two months ago I got my braces removed and they suggested Invisalign to keep my teeth in place. I really do take care of them. Had tonsil stones before braces but during that period they disappeared quickly and now they are back again. It makes me feel horrible tbh. I really thought they are gone forever.

Has anyone encountered such a problem like this? Is there is something to do with Invisaligns themselves? Really looking for an advice or some kind of tip&trick.