As long term SLF but no expert - I doubt it. Those that know will tell you the details shortly but my first guess is that it triggers in you:
- The knowledge that you cannot do anything about your surroundings and so you might as well relax.
or
- A protective mechanism, if you are sub-consciously worried about flying, your brain might think it easier to go to sleep.
or
- Getting to the airport on time is stressful, getting through check-in and 'security' is stressful, finding the remote gate is stressful ... time to snooze until the grub arrives!