Generally, if your door is opening on its own, it is due to a short in the transmitter. When there is a short in this the transmitter, it causes a signal to be sent continually. Even though you watch the door close completely, it will open back up because it hits the header bracket and reverses.
To combat this issue, start by taking the batteries out of the remote and just use the wall button. If that doesn’t stop the problem, then you need to replace the wires on the button. It’s usually one of these two things that will cause the garage door to open on its own after it fully closed.