Why does touch keyboard pop-up when I'm using*the pen as the "mouse"?

[Chris_NH]

I have a Surface Pro 3 with the latest slow ring insider software (Build 14986). I have equipped my SP3 with a SP4 keyboard and pen. When I use my SP3 with only*the (SP4) pen, in other words with the physical (SP4) keyboard detached or reversed, I often see the on-screen (touch) keyboard pop up when I tap a menu item or other type control with the pen. The touch keyboard appears even when text editing is not needed and it covers a large part of the screen until I close it by tapping the X (with the pen!). So I have two questions. 1) Has anyone else experienced this behavior? 2) Does anyone*know how to switch off this behavior?