Your flight is delayed or canceled. Airlines limit the number and size of carry-ons. If you are making connecting flights, one plane's configuration may be different from another's. You also pack in your carry-on whatever clothing you may need during the first 24 hours at your destination. The way things are, you're lucky to have a seat at all.
Airlines overbook seats, and it's legal.
For the rest of the huddled masses, the airlines certainly accommodate requests if they can, but make no promises. But you have to ask. This is legal. No, we can't repeat that enough. They expect some passengers to stand them up.