"CMunrock"......Your tire has a tread separation. A section of the tread separated from the tire carcass and air got in between. Once this occurs, the tire wears more quickly there, causing what you see.
The tire is junk and is highly likely to explode if used again. The separation can be caused by a bad quality tire or a sharp strike to the tire in that area, such as a pothole. You should always be careful how you park your trailer as not to stress the tire.
If the tire went bad, it is likely that the others will too. You should have them inspected. That typically involves lifting the trailer or removing the wheel so that it can be rolled and inspected on all sides.