Stuff like that is fairly common, especially if a tire is ridden slightly underinflated, or by a heavy rider.

However, it's not of any concern, since the rubber is just a cover over the tires structure. The only issue may be that over time, the bond between the tread rubber and tire body will weaken, allowing the tread to break off in pieces. That doesn't happen overnight, and progresses slowly so you have plenty of time to decide when you want to call it.

Those of riding high end tubulars are used to tread separation on old tires, and learn to glue it back down with rebber cement.