Haven't gotten into my 2nd GT barrel, yet, so I haven't seen it there, but definitely seen it with other cartridges - especially when swapping barrels. Basically, the very bottom of the case isn't getting sized down and eventually expands just a bit.
The fix for .308 case head size cases is actually surprisingly easy (at least, it has been for me, in the past). Get a Lee .45 ACP U-die (undersize sizing die), take the decapping stem out of it, and run your cases through it. It's a carbide die for a straight wall pistol case, so it's just a sizing ring - won't bump your shoulders or anything like that (which is something that *will* happen with cartridges that have a small base die - to full size the base, you end up squishing your shoulders back a lot, too). Then do a normal sizing die pass.