It's a Logitech G700 and I'm coming here after asking them about it. There's no option in the software that says use this button to move the window across the screen. It smells of a Win 8 "feature", but it doesn't sound like you folks are familiar with such a feature.
So, with all the pinning and tablet functionality, and the fact I've never seen this in Win 7, there's no such setting in Win 8? The window pins to the right, then it unpins then pins to the left side, then moves on to the other monitor.
It's been going on for months. "Recently" means I've been doing this for years and only had this problem with Win 8. I'm just not familiar with Win 8's new features and where they're hiding all the settings.
So again, you're implying that Win 8 has no mouse-controlled docking functionality, yes?
I have a Logitech M570 Track Ball and a Logitech M510 Mouse. On a clean install with no SetPoint software installed, all the buttons work with their default function. It sounds like something is remapping your button.