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.

Might be time for some new tubes......
Yah: The tube is on its way out...Don't wait too long to replace...
Thanks, I was afraid of that...
Don't turn it on again till you get a replacement tube, it could burn out a resistor.
Contact this guy for your tube!
Folks, you're not seriously suggesting that the OP replace just that one tube. Geez, they're six-year-old output tubes. It's long past time to replace ALL of them.
Jafox is trying to act as my "savior". Selling my quad would get me out of a project??? Contact me. If you need just a single EL84 till you decide, I may have something laying around (like a telefunken testing 50%)for really cheap.