My understanding was that the "boundaries" marked the point where the normal stabilization mechanisms in the environment wouldn't bounce back or where they were

My understanding was that the "boundaries" marked the point where the normal stabilization mechanisms in the environment wouldn't bounce back or where they were so kicked over that change rapidly accelerated. It was explained to me like putting salt into a freshwater pond. It can take some but the salinity eventually reaches a point (boundary) where the fish die and you can't really "go back" and take out the salt.