There are a number of mistakes that you can make as a lead climber, and back-clipping can be one of the worst depending on the climb and how high above the ground you are.
It’s a dangerous mistake that is common among beginner rock climbers, and can result in the quickdraw unclipping during a fall.
It happens when you clip the rope in a way that it’s running through the carabiner incorrectly. The correct way is to have the rope coming up from behind (between carabiner and wall), through the carabiner and out, away from the wall and then to your harness. A back-clipped carabiner has the rope running through the carabiner from the front, toward the wall, and then to your harness.
Even experienced rock climbers occasionally make the mistake, which is why climbers should always be double-checking their clips and correcting if necessary. Belayers should always be watching a lead climber’s clips, especially if the climber is new to leading.