Perhaps some other app you have installed? Decks should only drag if you're holding down the mouse/trackpad. You could also try the alternate Anki build on the download site, as I believe it's not affected by apps like Magnet.
I tried disabling as much as possible, but I am still experiencing it.
I was able to move closer replicate it though. It happens when the focus is
in a different window, and I click on a deck while the other window still
has focus. Anki gains the focus, and the deck goes into drag and drop mode,
even if I only single-tap (rather than tap and hold). I can try and make a
video if it will help