Actually, it's not a retirement, it's an option for resignation. F/As with 20 years of service who give 6 months notice prior to resigning are eligible for 8 passes a year.
There has recently been a big increase in requests to waive the notice period - mostly from flight attendants who do not want to return from voluntary furlough. But the 6-month notice is there in order to allow the company to replace the F/A. It can take up to 6 months to recruit, hire and train - the first new-hire F/A will not be on the line until sometime in March, and recruiting starts in November.
Retirement is different. For this "special" resignation benefit the notice period is there to encourage F/As not to resign without notice, (especially right before a busy travel period). Nobody likes to work over the Christmas holidays, but probably the reason the company is recalling everyone now is that they need the extra F/As for the holidays. Many of the F/As in this round of recalls, the most senior, have not worked since 2002. Many don't really want to come back.... but they also want the passes, so they're feeling forced to come back and fly for 6 months.