It is driven by P&E vs P&E.
To clarify, there are two things that push a system. The first is players running sites they find via ALT+P. The second is NPC fleets being routed in the system. This can be done by both Triglavian NPCs and players.
This is also why the Caldari State have lost over eighty percent of the systems that got invaded, and why nearly ten percent of their space is now either a Triglavian Minor Victory or a Final Liminality system. If left unattended by players, Caldari systems are automatically pushed into FL or TMV as the Caldari rats try to hit fast-moving, low sig ships with missiles intended to be used against battleships, and use all their ECM on a single target instead of spreading it out, on top of having resist holes. They’re effectively unable to mount a resistance against the Triglavian NPCs because CCP refuses to deploy their hybrid weapon ships.
Once a system hits Final Liminality or Triglavian Minor Victory, it is, currently, irreversible. You have to attack/defend it before the bar locks.
To avoid repeating myself, see here: