Davey

When you take a hard turn, you load up the outside tire but not the inside. Then to drift you overpower the car, and because there's more grip on the outside than the inside tire, you will tend to spin the inside tire and not the outside.

The more you use the viscous LSD (like spinning the inside wheel, but not the outside) the more the oil internal to the limited slip unit heats up and breaks down.

Just like your motor oil it becomes "runny" - and since the viscosity is what makes it work to begin with that means it has become less effective.