It seems totally random. Did you miss the PHL to MCO ding the other day?? I know they had one recently, as we also fly out of PHL. There have been probably 2 of these Philly ding fares in the past month. We booked one a few weeks ago.
In all honesty, just book the "internet advanced purchase ticket" now to make sure you get the flights you want. SWA is running $49 each way, and the ding fares have been $43 I think. If there are no internet fares available for the flights you want, there will be no ding fare available either, so don't wait.
We actually had booked a Ding fare, and then switched to the internet fare to get better flight times. It added $22 to our entire trip for 2 people.