Sometimes you just have to hit a permanent geological feature in the right way with a relatively small force for big things to happen. During WWII, the Soviets were bombing a bridge in Finland and missed their target by a wide margin, hitting the side of a nearby hill. The hill, which no doubt had been there for eons, happened to be of such an inherently unstable nature that the bombs caused the whole hill to slide down into the river, completely covering the bridge.
"I suspect that your idea that this upwelling being 'on a knife's edge' is correct. However, just because a small amount of energy was put into drilling compared to what was released doesn't mean that they're not related or even necessarily that the system was 'on a knife's edge'. For all we know the exploratory shaft could've breached an impermeable layer a mile thick; such a situation would actually be very stable until the breach. The relationship between energy in and out has nothing to do with the stability of the system. It's possible to put a relatively small amount of energy into a very stable system and get a huge return."
You are correct, but that doesn't mean that it is likely. I believe that in the vast majority of cases, the triggering energy would have to be applied in a very precise way.