Here’s a picture, a little blurry, of a spark plug showing carbon tracking on the outside of the porcelain insulator. The plug is fairly new.
Would anyone like to guess what caused it?
I would say cheap or bad plug wires.
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That occurs when the spark runs down the side of spark plug instead of to the gap.
Change the plug wires or coil boots.
Tester
The carbon track was existing in the old wire when the plug was installed.
Autolite is not my favorite but it was not a plug or wire defect.
Plugs and wires are brand new.
New wires were installed with the new plugs.
Water in the spark plug well?
I don’t see any mention about replacing wires.
Tester
Crack in the porcelain?
Contamination, your technicians will need to wear hair nets.
OK, it was a trick question. I cleaned the spark plug before taking the picture.
Guy’s car was running rough and the check engine light was on. He went to Autozone for a code read and found a misfire, told him most common cause was plugs and wires. The clerk sold him spark plugs, a wire set, and a small tube of anti-seize. Guy goes home, installs the new plugs, packs the wire boots full of anti-seize, installs the wires, then wonders why the car runs worse than before.
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try cleaning them with break spray and then see if it happens again
Well, maybe the one that wears a man-bun, but the bald guy surely gets a pass. 
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Nope, not cleaning anything. I work at a shop and we warranty our work for 3/36. It was new plugs and new wires or take the car and fix it yourself.
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Agree with @asemaster , new parts if you provide a warranty. For DIY I would clean it but only if the plugs were easy to get to. Some plugs require major work to access and I would use new parts then. Besides it might now work after break spray, might be broke then 
This is what some people don’t get when they complain about shop prices and practices. You can try shortcuts DIY, but any decent shop that stands by its work has to fix it right.
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