So I'm at FRA looking out on two 777's (ANA and Ethiopian) and noticed that as the ET jet was towed in to park the turbine I could see (starboard) was rotating clockwise, i.e. the direction you would expect during flight to draw air into the engine. Almost as soon as it parked the turbine was rotating in the opposite direction at a fairly constant but low speed (10 rpm or so). I then looked at the ANA aircraft and the port engine was doing the same (couldn't see starboard). Both have now been doing the same for a while now. They both seem to have GE engines.

Can someone explain why this happens? Is it specific to GE/Boeing/777 aircraft?