Schwalbe Marathons:
This has happened to my front tyre on the commute bike, well before the tread has worn out. It's not an arduous commute, 4miles each way, daily, all on tarmac roads/paths, some short sections of cobbles.
The tyre is always kept pumped up to 60 - 85psi. And it's not due to the front tyre having less wear, this also happened to my rear tyre previously - the tread started to separate before it had worn out. I thought that was just a one-off, but it seems not...