Would you say that is related to the fact that the suspension needs to move farther for a bump as lean is increased?
In a simplified model that comes directly from mathematics/ trigonometry/ tangent function.
That is why:
At small lean angles lateral force grows slower per each additional lean angle degree.
At big lean angles lateral force grows faster per each additional lean angle degree.
For example, lateral force calculated directly (without contact patch offset) from lean angle with tangent function:
At lean angle 25 lateral force is tan(25 degree) = 0.466 G.
So you need (at least) coefficient of friction 0.466 to keep that lean angle.
At lean angle 26 lateral force is tan(26 degree) = 0.488 G.
So you need (at least) coefficient of friction 0.488 to keep that lean angle.
Diff is 0.021 per one degree change in lean angle (from 25 to 26).
At lean angle 49 lateral force is tan(49 degree) = 1.150 G.
So you need (at least) coefficient of friction 1.150 to keep that lean angle.
At lean angle 50 lateral force is tan(50 degree) = 1.192 G.
So you need (at least) coefficient of friction 1.192 to keep that lean angle.
Diff is 0.041 per one degree change in lean angle (from 49 to 50).
As you see, at 49...50 deg lean angle the change rate of lateral force is almost double when compared to change rate at 25...26 degree.