Why do parked jet engines spin backwards?
 

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?

Wind and direction of wind. They free-wheel without power. Even a big turbine can be turned by hand....

Were they both facing the same direction? Was it breezy outside?

I did wonder that but the turn rate was pretty consistent. Usually breezes come and go.

Yes, and don't know. What surprised me was that it didn't seem to take more than a few moments (I was distracted by a G&T) for the ET engine to have reversed direction and reached a constant turn rate.

The weight/momentum of the spinning components is enough keep it going for a little bit even if the wind dies off momentarily.

As others have said, it's the wind. It's a useful demonstration of how freaking good the bearings on the engines are, though. I've seen 777 fans start turning backwards merely from the speed of pushback. No joke.