This is probably a formula to model what is called "Adverse yaw" in aircraft dynamics. It is a very popular factor - along with Dutch Roll - in terms of fixed-wing aircraft design and control, as it causes the vehicle to make an undesired movement.
In terms of a quadcopter here is what I think: When you perform a "roll" maneuver, the propellers which contribute to the roll angle in the desired direction will have more speed than the others. That way the quadcopter is going to perform the roll. However, these higher speeds will also cause those propellers to produce more drag (recall the thrust and torque equations). Because of that reason, quadcopter may tend to perform yaw movement in the opposite direction according to other angle inputs (combined with pitch angle input).
As the roll angle increases, the adverse yaw may be expected to increase due to higher speed difference - therefore drag - between the propellers.
This formula might be a simplified version of complex aerodynamical interaction between the vehicle and air. They might have approximated it from a real-life data. Maybe if you do some research on "Adverse Yaw", you might find similar equations.
This may not be the complete answer you were looking for, but I hope that it would direct you towards a more clear way to understand the formula/interaction.