I often will hold a window's title bar while reading something, and every once in a while the window will become maximized while I'm holding it.
I can't totally reproduce this, so I'm not sure the exact behavior that's causing it. I'm not hitting any other keys while holding the window, and I believe I'm holding it still. A bit of looking on Google isn't yielding me results. I hold onto windows by habit so this has become pretty annoying.
macOS High Sierra 10.13.1