I am only guessing because I used to own tube amps eons ago..I would gather the gas from that particule tube escaped and the forth started early.
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Tubes have a cathode that emits elections. It is heated so it will emit electrons more efficiently, and finally it is also treated with a sort of paint that improves its efficiency even more.
It is this coating where you have most of the arcing problems.
If the tube has high voltage applied to it during warmup,
If the tube took a hard ride in shipment
If the coating was not properly applied (**very** common these days)
If the coating has aged due to use
Then the coating can fall off of the cathode in chunks large enough to cause arcing in the tube. Sometimes the tube is permanently damaged, sometimes the arc can be cleared and the tube might give normal service for a good while longer. Usually knocking on the top of the tube with no B+ on it is the way to try to clear it. My rule of thumb is that if the tube continues to misbehave after that then I replace it.