When the focus is one the desktop itself, the auxilliary mouse buttons (i.e. additional buttons on the sides of the mouse) switch desktops. I have Microsoft IntelliMouse Optical, which has an additional button on the left side and an additional button on the right side. The former switches to the previous desktop, while the latter - to the next.

Is there any way to disable this behavior?

I looked through settings, but I couldn't find an option that would control this. Desktop Manager has an option for mouse wheel switching, but not for auxiliary mouse buttons.