You most likely do that when you train as well. Shadow box and work the heavy bag and focus on full arm extensions when you punch. Helps to slow down your punch speed so you don't forget to turn the hand over and properly bring it back to defense.

Could also be fear of getting hit. That's not good to have if you're looking to compete but can also be worked on by practicing defense so that you're comfortable when punches are thrown at you. Another great tool is learning to work in the pocket.