You can search the ExpressJet threads and find the details but,

CBA 2007 included a "fragmentation policy" that provides for the transfer of pilots with airplanes within the Skywest system with seniority. The CBA specified "sale, lease or any other means".

In the 2012 TPA, after the merger with xjt, the "lessor clause" was added. This means that aircraft returned to the bank are exempt from the fragmentation policy. This provided a pathway to transfer airplanes from one entity to another by returning the airplane to the bank and re-leasing to another entity without transferring pilots.

This happened under a previous MEC and CNC and after exhaustive attempts to find out how and why this happened, and what was received in exchange, there has never been an explanation.