I have a '98 Millenia and had sparkplugs replaced about 8 weeks ago by AAMCO transmissions. #2 plug blew out causing my car to stall. I had it towed and fixed at a Mazda dealership. They explained all the sparkplugs were loose, #2 blew and had to chase the threads in the cylinder.
AAMCO does not understand how my Millenia could run for 8 weeks before this occurred and of course does not want to reimburse me.

I have read that if you don't properly tighten the plugs, they will slowly work their way out and blow.

Can anyone help me better explain this to these D-rated mechanics?

Does your sparkplugs have compression rings or not? A compression seal plug requires a different spec. to torque compared to a taper-seal plug..I'd ask for proof that the plugs were PROPERLY torqued.