I just found it ironic -- and possibly suspect? -- that 1 week after this XKCD What If? article (http://what-if.xkcd.com/40/), something like this happened. Could the XKCD article possibly have inspired the method of execution?
No, i think that a) this was planned weeks or months ahead and b) the idea that explosive force is increased by putting your chemicals inside a high-pressure container is explosives 101, so to speak.