Probably not a malfunction. Garage doors are designed to go back up if the encounter resistance; your door is probably perceiving the ground as resistance. You can fine tune where the door stops when it is closed to address this. It's usually some nobs or screws on the motor body. Just adjust until it closes without rebounding.

Otherwise I get this periodically when the one of the wheels starts to catch in the track. Lubricating the track makes it stop.

the opener has adjustment screws for travel up/down and force up/down. i usually pull the release handle and just watch the trolley go up/down as it is easy to figure out the proper movement

Its almost always the stuff you mess with. My guess is you’ve misjudged where the door was and the door is still trying to close further when it feels the resistence of the ground. You need to reset the closed position on the door, simple as that. Assuming if you can replace the stripped gear, you should be able to find these instructions as well?

I agree with the above. Find the instructions for the opener. Either the limit switch is set too low or you have an obstruction. Look to see if the door is meeting the floor evenly. Also check to see nothing has bent the track. I've seen the bottom roller binding on such deformity in the track trigger the door to think it hit a blockage.