Thank you Arlo for thinking along.
I use the Pageflip Firefly to flip the pages.
It indeed happens when I want to turn a page in a song. When I unintentionally press the pageturner to much and the song changes after the last page, the associated BeatBuddy drums change via Midi to the next drumsong and the current drum rhythm stops. During a gig this is of course not nice.
So I thought maybe I have to enlarge every song in SLM with 1 or two blank pages to prefent unintentionally go to a next song and thus BeatBuddy-song. But maybe someone recognizes the same problem and has a solution for it.