What happened to my EL84 tube?

I was listening to my Jolida integrated last night and something strange happened when I turned up the volume. One of my EL84 output tubes started glowing bright orange and the left channel started crackling. I powered down the amp, let it cool and removed the tube. I put the tube back in, powered it up and everything sounded fine.

Does anyone have an idea of what might have happened? The tubes are all stock and this amp is about 6 years old.

Thanks,
Tim