Answer: This happens because the mechanism by which increasing Solar luminosity causes atmospheric CO2 to decline is temperature dependent.
Explanation: Increasing solar luminosity raises the temperature of the planet, increasing the rate of weathering, which exposes crustal rock to the atmosphere. Atmospheric carbon dioxide reacts with that exposed rock to form carbonate minerals, which removes the CO2 from the atmosphere.
This in turn lowers the greenhouse effect and lowers the planet’s temperature, maintaining an equilibrium temperature despite the increased solar luminosity.
A negative feedback loop.
But because solar luminosity is in fact greater, Earth cannot cool to BELOW its prior equilibrium and thus a snowball Earth cannot really occur by this mechanism alone.
But once CO2 is sufficiently exhausted from the atmosphere such that further lowering it levels no longer decreases the greenhouse effect, the temperature buffer effect ends, and Earth’s temperature starts to rise.
Once it rises past a certain point, carbonate minerals are no longer stable. Exposed bedrock no longer reacts with atmosphere CO2. Instead, the REVERSE happens. All the previously made carbonate minerals now decompose and RELEASE CO2 back into the atmosphere.
The greenhouse effect now increases despite the Earth already being warmer, making it even warmer.
A POSITIVE feedback loop.
Which in turn makes the carbonate minerals even more unstable, which increases the CO2 release rate even more, which increases the temperature even more.
Thus a runaway greenhouse effect commences.