It should come as no surprise that under New York law there is a great deal of overlap between non-compete and non-solicitation agreements and breach of fiduciary duty claims, particularly as they relate to employees who leave to work for a competing business.
The reason for this is relatively straightforward: by dint of the trust that was placed in this employee while they worked for Employer A, they were often charged with responsibility for cultivating and maintaining client relations with Employer A's clients. And, as is typical for these claims, Employer A went to the trouble of reducing this to writing, in the form of a non-compete and/or non-solicitation clause in an employment agreement, which not only purports to bar this employee from poaching Employer A's client base in the event that the employee quits or resigns, but also seeks to bar the employee from working for a competitor for a certain amount of time, usually a year. (For more on this topic, see "When a Non-Compete Is Enforceable Under New York Law").
And it is this employee's purported breach of the heightened level of trust which also forms the basis for a claim that the former employee breached the non-solicitation clause of his employment agreement (a/k/a breach of contract). In other words, although the elements that need to be proven in a breach of fiduciary duty claim are distinct from those required in a breach of contract cause of action (see, "What You Must Prove to Win a Breach of Contract Case in New York"), it is the same set of facts that gives rise to both claims.