There is a need for outside force, there has to be for everything on earth. It's always equal and opposite reaction to action. "ALWAYS."
There is an equal and opposite reaction, but it comes from the walls of the container itself, not from any outside force.
Okay, let's go back over it: everything used to make pressure vessels has a certain tensile strength; a certain amount of strain it can handle before it breaks. My examples were a steel cable lifting a large weight, and a balloon clamped so that weights could be hung from it. In both these examples, there is no opposing force other than that supplied by the cable or balloon; they are solely responsible for carrying the loads placed on them.
Pressurising a container made of steel, or blowing up a balloon, is not that different to hanging weights off it. You are simply placing the sides of the container under tension in the same way as hanging weights off it. And just like you can only hang so many weights off it before it breaks, you can only pressurise it to a certain point before it bursts. This limit is
solely governed by the tensile strength of the container, and has
absolutely nothing to do with the outside environment.