Why always these problems with seniority mergers in the USA?
Back in the days of regulation mergers were done with consideration of the "career expectations" for the two employee groups. Northwest and Republic employees, to pick an example, had different "career expectations". In this day and age the idea leads to arguments about who is bringing more to the merger.
This is aggravated by many airlines having had hiring cycles that were out of sync, so even very similar airlines can get very messy when looking at straight date of hire integrations. America West and USAirways for example, where America West had Captains with later dates of hire than furloughed USAirways pilots.
All of which set the stage for greed to come front and center.