Thanks to Youtube, I have now seen some of it. I’m not sure there’s enough data to answer the question, but I have a few ideas.
First, I don’t think what you see is what you get. We see a flat-ended cylinder, which is not an ideal container to hold pressure. And from the voice-over, the tanks are rated to hold 150 psi. So I’m guessing that inside is a dome ended cylinder to hold the pressure, surrounded by insulation, and then a thin outer housing, which is what we see.
Secondly, pressure vessels are usually designed to be tough, i.e. they bend before they break. So the pressure vessel inside isn’t going to shatter into frag like a glass bottle. It’ll pop a seam, or even fail at a thread as you suggested. So it might be only the outer shell that fails at the bottom and gives the rocket effect.
One reason the outer shell might always fail at the bottom is a subtlety of geometry. When the steam blasts blasts out into the shell, it’s trying to inflate it like a balloon. The wall is a cylinder, the top is flat but it can dome upwards, the bottom is flat but it’s constrained by the ground. So the bottom seam remains at a right-angle for a fraction of a second longer, which makes a nice stress concentrator.
Or maybe I’m over-complicating things. Maybe the bottom of the shell is simply crimped on, while the top is more substantially attached, for obscure manufacturing reasons.
Another alternative is the effect of orientation and headspace. Presumably the pressure vessel isn’t entirely full of water, so you have a headspace of pressurised steam at the top. Boiling is a vigourous process so there is agitation, steam bubbles travelling to the top and bursting. Maybe a shockwave effect, or something else to do with this asymmetry, means the bottom dome always blows off first.
All WAGs. More investigation is needed! We need before-and-after dissection of a tank. More burst tests with a tank upside down, a tank on its side, a tank suspended off the ground by a collar around the cylinder…