Both KZG and KZJ are the blacksheep of NCC/UCC (or were, rather - both series were discontinued years ago). Two exceptionally bad series from an otherwise good catalog of capacitors. It's commonplace for both series to produce an excess of hydrogen gas with their external leads still intact, without ever having been placed in a circuit before, just like Fuhjyyu, GSC/Sacon/Evercon, YEC, Rulycon, etc. What's interesting is that ALL the 3300µF 6.3V KZG died on their own but the 1500µF 16V KZG still "appear" okay.
I'm not surprised that those CapXon LZ or GF? series capacitors died on their own accord either. CapXon is a truly awful brand. I would be interested to know how old they are, datecode wise, however. It isn't at all odd for liquid electrolytics to bulge of their own volition. You can only cut so many corners with respect to the electrolyte and other materials before it simply won't last, even in storage (unused). There are thousands of vastly complex electrochemical reactions taking place in an electrolytic capacitor. Good capacitors have the right oxidizers and neutralizers to keep these reactions to a minimum over the course of a very long time. Heat and stress will only act as the catalyst and accelerator of a volatile chemical reactant that will eventually occur anyway.