Two contracts… Really?
Yes. Every employee has two contracts. The employment contract written down and filed away somewhere, and the other one that you, as a manager, ignore at your peril.
It is what the employee thinks is reasonable, what they think they have been promised, and their expectations of how they think things should be done. It is often called the psychological contract.
Are you considering changing something, or wondering about the strong reaction when you did change something?
Something like changing hours of work, changing where people can park, putting in a paid for coffee machine, changing the notice required for holidays.
If these changes violate what people expect, or what “should be”, then people will resist, even if the change seems trivial and within your remit to make.
So tread carefully when making changes, and consider the psychological contracts all employees have.
PS This concept of an implicit contract also occurs in other relationships outside of work 
My best wishes, Paul
.