Lantern10 has it.
Air travel is a microcosm of society. The pressures of overpopulation are starting to have a noticeable effect in many societies, and the crowded interior of an aircraft represents as overpopulated a space as you could get.
Less room= more stress, and also equals larger numbers of people at the extreme ends of the old bell curve are likely to be nearby,
There's a fair bit of "ground rage'' at large in the world, too. Seems to be increasing. And some of it is hideously extreme.