Hard to say. The first and easiest thing would be to check the tubes and make sure they're secure in their sockets and no visual signs of problems. Beyond that, a faulty capacitor or bad connection somewhere would be a suspect, especially if it continues. Do you have anything else in the loop? Did you build the amp? Without being able to reproduce the same symptom makes it hard to troubleshoot. But if it's a component like a cap, connection or even transformer it very likely will continue to get worse.

Also, what's the chance of voltage spikes in your practice room? Although it would have to be a heck of one to cause that kind of reaction.

Are you certain that your soldering is solid? I only ask not knowing how skilled you are.

I've started doing more repair work on pedals pedals (some DIY, some modded name brand stuff bought off ebay) and I'm totally shocked at some of the awful soldering work that I'm guessing someone thought was acceptable. (little crumbly balls of solder barey melted onto the contact, etc.) These are all problems waiting to happen. Something could work for any length of time until it finally fails due to vibration, thermal cycling, etc.