Why are low-flow toilets more likely to overflow?
I have seen something similar happen in an area with hard water.

Over a period of time limescale builds up around the water inlet into the toilet cistern.  This in turn leads to the water flow not shutting off completely as the ballcock rises.  The limescale keeps the valve fractionally open, enough for water to keep flowing slowly.

Usually this water should just go down the overflow and not cause a flood.

If this is your problem, it is easily cleared by breaking up the limescale in the cistern.