Right Click Navigation is a new feature of 3.5. One of the default ends of the pen button is right click, so naturally this would rotate.
By default, the pen button double clicks on one end, and right clicks on the other. The right click end of the(default) pen button will still bring up the right-click menu if you simply click (and release) it, but I notice if you hold it down, it will rotate instead. The double click end seems to simply rotate.
You can probably easily fix this by simply mapping the spacebar as a keystroke to the pen button. Then it should bring up only the right click menu, unless there is unexpected behavior…the right click menu and the space bar have some unintended quirks at present.