The Limit nuts are out of adjustment. This can be caused by the chain jumping the sprocket, usually when something gets caught in the chain or it has not been adjusted. It can also be caused by a cracked limit nut. The limit switch and nuts are located at the top of the operator (under the cover) and has a lid held on by a single screw. Loosen the screw, and remove the lid.
Inside you will see two cogs on a threaded shaft with slots cut into them. These slots are there so the lever will keep the nut from spins and allow it travel down the shaft as the shaft turns. The limit nuts eventually contact the limit switch, stopping motor. Look closely at the sides of the nut to make sure there are no cracks allowing the limit nut to spread open and slip on the threaded shaft. You may have to clean them to see it.
If there is a crack, usually found between the brass nut and a slot, you will need to replace the limit nut. Go ahead and do both nuts at the same time as it is a challenging process and you will want to do it only once.
If the nuts are not cracked, simply press the bar holding the nuts in place back with a screwdriver and spin the nut towards the limit switch until the gate stops about one to one and a half inches away from the post. The gate stopping point can vary even more than one and a half inches on slopes so if it starts happening again, give it even more space.
Finally, a new limit nut may look different than the old one. The old ones with the brass nut encased in plastic had more cracking issues than the new all plastic nut shown here.