It would increase with low democracy / high corruption / high military spending / low general security rating, as well as number of turns spent with emergency powers (plus some cooldown so you can’t just cheat the system by somehow staying right on the verge and triggering it every other turn or something).
Basically, right now, there’s very nearly no downside to having emergency powers and it’s one way to make the game extremely easy, knocking out expensive policies left and right.
To truly make that a balance, the chance gotta be able to climb to fairly decent levels even without the other modifiers (besides turn length), but those four would speed it up immensely.