Internal arcing typically is due to one of two causes. Over-voltage applied to the tube by the amp circuity, which seems implausible in this case. Or loss of voltage hold-off capability internal to the tube, which implies the tube is no longer fit for duty. This later case seems the most likely.
BTW, the inability to hold off voltage can be due to several things including, some physical change to the electrodes, like breakage and movement closer together, a coating pealing and curling (moving closer to another electrode, something generating particles internally that charge up and then fly around due to electrostatic attraction, etc. or a partial loss of vacuum (increase in pressure). The increased gas pressure inside the tube ionizes, creates plasma, becomes conductive and then supports arcing. The increase in gas pressure can be due to a slow vacuum leak, or can be caused by heating a surface inside the tube that then releases gas. Both sources of gas are counteracted by the silver getter coating who's job it is to chemically absorb excess gas thus preserving vacuum. Eventually the getter's capacity to absorb gas will be exhausted and the gas pressure rises uncontrollably, ending the life of the tube in a manner similar to yours.
I work in the tube industry making microwave tubes, hence my familarity with this type of thing.
