See Bug 475704 for the investigation that led here. Pen buttons are re-bindable. However nowhere is it shown what they are bound to by default, so you don't know. Apparently there are pens with buttons that do different things; some buttons emit different kinds of clicks when pressed, while others toggle the pen between modes (e.g. drawing mode and eraser mode). However nowhere is this shown in the UI, so the user has to guess at what the buttons do, figure it out through trial and error, or be a KWin developer and scrutinize WAYLAND_DEBUG logs to figure it out. It would be much better if the buttons' current functions were shown in the UI, and the UI made it clear that binding actions to those buttons would amount to overriding the default actions.