It's always been a mystery for me while creating drop shadow effects that one layer changes the angle of its shadow automatically if I change the angle of the other.

For example: in Layer 1 I am setting the drop shadow angle as 0. In Layer 2 I want to set its angle to 90 degrees. As soon as I do this, the first layer changes its angle to 90 degrees.

Why is it dependent and how do I remove this link?