? about spark plug condition

I have been running a MSD billet distributor and 6AL digital box on my race truck for a number of years now with no issues. Recently I had a problem hearing misfires in the header tubes so it sounded. First thought was to check my plugs and check over the ignition system as I hadn't done that in a while. When I pulled the plugs I was surprised to have 6 out of 8 plugs with the center tip missing half way down in the ceramic. The rest of the plug was fine as outer tips were ok and the ceramic was fine with proper color. My question is what condition would cause that if not normal wear? I put in a new set and what a difference. Would like to solve problem if necessary to not tear these up.

I changed out the plugs in my ex's '07 Nitro SLT at 50K and they was the same way. Had no issues before or after, this reminds me I need to check them again.