A friend of mine is fond of writing while flying. It’s not a habit of mine, but I like the idea—the not here anymore but not yet there either is so much like writing itself.
There is also this: When traveling, your body usually arrives earlier than every other part of you—just how much earlier is directly related to how fast and how far you went. When you’ve put pen to paper or tapped a rash of keys (typing is, notably, much faster than handwriting) it can take awhile for your brain to process what you’ve just drafted. The physical thing happens whether or not the rest of you is ready for it—whether or not the rest of you can keep up.