I've seen that when debris is present in the combustion chamber. AFAccord had some shop fill his IAB shaft holes with JB Weld, some of it broke free and lingered in a cylinder - he broke a couple and scarred the chamber before it passed out the exhaust. In fact, he spent an afternoon making laps in front of the shop and stopping to check his plugs as the JB Weld bits got smaller and made tinier cuts in the ceramic (after they'd stopped breaking it).
But, yeah. What brand plugs? Have you changed anything recently?