Nintendo Switch controllers can sometimes have failures in their joysticks. This results in movements that are recognized by the controller, even if you don't touch the joysticks! Nintendo offers to repair your Joy-Con if you encounter the problem, but this means that you must send the controller to the after-sales service and the process can take several weeks before being fixed.
If you are a bit of a handyman, you can choose to change the joy sticks of your defective controllers yourself.