The most likely reason for the door to reopen before reaching the closed position would be something triggering the safety sensors. On most models this will also cause the overhead lights to flash.
Those sensors are intended to prevent the door from closing accidentally on anything or anyone so if they sense something in the way the door will reopen. The system primarily triggers if something blocks the site-line between the two safety sensors near the floor on either side of the garage door.If you have a loose sensor mount or a mis-positioned sensor then the vibration caused by the moving garage door might cause the sensors to ?lose sight? of one another and read this as something blocking the signal. That would cause the door to reopen. The images below have more information that will apply to most Craftsman models.Another possibility is a pinched wire, or one with a poor connection, between the sensors and the overhead unit.The safety system can also be triggered when it notes an unusual amount of force required to close the door, since that could also be something under the door. Binding or drag in the rails or door lift could cause enough added resistance to trick the system into triggering for this reason. You can often find issues there by using the release rope to disconnect the opener and then checking the movement and balance of your garage door by hand. Often adding some lubrication is enough to make a big difference in those situations.Or, there could be an issue with the limit switches themselves. A poor connection in the mechanical screw-adjust type or a problem with the memory on the electronic adjust type. And, sometimes an issue with too much freeplay at the gear assembly where the limit switches attach makes it hard for the limit switches to determine what position the door is in. Or something out of place at the RPM sensor might cause an issue with the control board confirming door movement. Any of these could cause the door to reverse open.The links below have some additional information that should help further for most models. If not, you may want to consult a professional for further inspection to be sure the important safety systems of your opener are operating properly. Even a small opener has enough power to do a lot of damage or cause serious injury if it closes at the wrong time.Or, feel free to submit another question here with more detail and include a complete model number and we will do our best to provide more specific information to help. You can also use this blue Owner's Manual link to find the manual for your particular model online at Manage My Home which will have troubleshooting information specific to your particular model opener.