What this claim addresses
It is aimed at conduct by someone other than the employer that caused a job, an offer, or an engagement to end. The situations that recur are a former employer making statements to a new one, a competitor threatening to enforce an agreement against a company that just hired you, a third party pressuring a client to drop a contractor, and in some circumstances an individual inside the company acting for personal reasons rather than for the business. An employer ending an at will relationship on its own is usually a different question entirely. Whether particular conduct crosses from ordinary protection of a business interest into something actionable depends on state law and on the specifics.
Documenting the outside push
The sequence is what carries this. Save the offer or engagement letter with its date, the communication withdrawing it, and anything the employer told you about why. Get that explanation in writing if you can, because the account given before lawyers are involved is usually the most candid one available. Collect recruiter correspondence, the agreement that was being waved around if a restrictive covenant is part of the story, and the names of anyone who heard the conversation. Note what the position was worth and what you have done since to find comparable work, since that question arrives early.
How these matters proceed
They are often brought alongside a defamation claim where false statements were made about you, and alongside contract claims where an agreement with the employer or the former employer is involved. Where a former employer is still contacting prospective employers, whether some form of early relief is available becomes the pressing question, and that depends on the facts and the court. There are filing deadlines, and they are not uniform from state to state or from one kind of claim to another. In a first conversation we look at whether the conduct is something more than a former employer answering a reference call, and whether the loss can be traced to it.