It would be interesting to know the fare basis code on the ticket you had. Some fares are time verified/time specific and if you fly outside that window, an add collect applies. I've experienced this on other carriers as well. However, if you were not on a time verified fare/time specific fare, then this is concerning.

I think we all know that all of Southwest's discount fares are, in effect, time specific. Absent weather or mechancial problems when they waive it, standby on a discounted WN fare requires upgrade to Y.