Static constructor not called always

I have partial class that have a static constructor described in one of its defention and In the other defenition of the static constructor a normal constructor is defined. My test cases are mocking this classes. When I run the test individually the mocked statement  Isolate.Fake.Instance<StaticConstructorClass>() is not calling the static constructor and when I ran all the test together the same statement make a call to the static constructor. Could anyone explain me this unpredictable behaviour?