The plumber pulled out a lot of linty fibrous material that may have worked itself down the open drain over time. It may have come from a nearby opening in the wall, possibly insulation, but really hard to tell. It's also possible that there was something in the coil drain pan initially, when my new HVAC unit was installed earlier this year. Anyways, I sure hope it's a long before it clogs again, since getting to it is a real pita and I could not fix it myself. The plumber actually had to construct an extension rig out of pvc pipe in order to insert the snake. It took him over an hour to get it done, whereas it would normally take less than half that time if the drain were more readily accessible. It was really nice that he stuck with his initial price estimate of $125.