I have seen a few caps break like that but it was usually in a street application and a non-vented cap with spring clips, like a Mopar. In those cases it was fuel vapor making it's way into the dist. from a super rich condition or a non-working PCV. If it was not struck and broken as mentioned, I would check crankcase ventilation. I usually drill several vent holes in all my race caps to prevent this and any ozone build-up.
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