If you're punching and your fist lands weird, your wrist can bend, and with all your force and bodyweight going forward, you can hurt your wrist. Surprisingly, your wrist can bend either up, like a palm thrust injury, or down. (Note: I know how dumb this sounds. That was the point.)
Furthermore, throwing one's hand straight forward with the fingers/wrist bent back and up is both leaving them in the ideal position to be bent back unnaturally, and supplying the force in the necessary vector to bend them back. All it would take is in an object - say, your opponents hands or head - intercepting your hand at the fingers or the top the palm to violently bend the hand back whilst throwing the palm heel thrust.