This happens on my Brera sometimes as well. Traced the problem on mine to an over-sensitive resistor-type-thing in the motor circuitry. It's designed to prevent fingers being crushed or the motor blowing when the window closes on an object. Sometimes, when the door is closed, the air pressure increase or vibration through the frame is enough to fool the system into thinking it should drop the window halfway. Annoying and difficult to fix. Mine only does it every few months, so I just live with it.