It's not the intention of the maker to design it to be explodable or rather implode due to it being vaccum tube. Some time one-off freak case or due to abuse or exegerated by some hyper human.
Same as CRT based television. There is always a risk of implosion, but is always caused by someone's "stupid" action.
For tubes, if you want to go technical. Some defective or worn out tube may become internally shorted during use. So in these cases, the shorting spark may be big enough to be alarming. Worst case it could blow the other components in the amp, rendering it kaput!! I have seen this, but it's internal of the tube only.
A faulty amp could also cause the tube to "red-plate", become overheated, leading to eventual failure. I have never seen a tube overheat till it explode, due to vaccum, imploding may be possible.
So it's best to use reputable tubes, and follow a timeframe to replace your worn out tubes.
Anyway such questions are not necessary. It's like asking: Is it still possible to get Aids if I always wear condom and frequent prositutes?