It's not just airplane seatbelts. I used to wear a G-Shock, and that thing always got banged around on my car seatbelt. I used to use my left hand (I'm in the US, so the driver's seat is on the left, and the seatbelt is to my left) to both get the belt, and buckle it in. because of that motion, since you have to pronate/supinate (sorry, i forget the right term) in the middle of the motion, and/or since you are reaching for the belt buckle blindly, I used to always scratch my G-shock.
Now that I have the Apple Watch, I am much more careful, and reach for the buckle with my right hand, decreasing the chance of banging it on the buckle when reaching blindly.
For airplane seats, the buckle usually sits in the middle of the lap, and people usually wear seatbelts much more loosely on airplanes for comfort (as opposed to car belts which are to your right/left and tucked away behind the body). Since they are loose and in the middle, it is much more likely you will hit your watch there.