Let's not leave out our friends at Mesa:

$50,000 bonus with $22,500 paid out at the completion of IOE and $20,000 paid at the end of year three if a pilot has not upgraded to captain. Additional $2,500 sign on bonus for CL-65 rated pilots who choose the CRJ. $5,000 additional for pilots who are type rated on the CRJ, E175 or E145 and have a minimum of 500 hours of part 121 flying in type.

Seriously, it costs your junior people virtually nothing to jump ship for bonuses, even at Mesa. So what if they lose nine months seniority, if they go to a place they can upgrade to Captain and get that TPIC sooner? They are going to lose that seniority when they go to a major anyway.