This sounds like an issue with your mouse, its drivers/settings, or your mouse technique, not with SketchUp per-se. That is, for some reason SketchUp is receiving multiple mouse input events when you press the button. Check what the system settings for the mouse are, they may be interfering with SketchUp. Can you borrow another mouse to see whether the problem is with the hardware? I don't know why you see this with newer versions but not with SketchUp 8 - perhaps they tuned up something in the GUI that used to drop almost-simultaneous mouse inputs...