Precocious sprouting is common in Clivia, Crinum, and most of the fleshy-seeded amaryllids, e.g. Amaryllis, Brunsvigia and Strumaria. Even though Hemerocallis seed are not plump and fleshy, they mature with a good bit of moisture in them, so it isn't that surprising that they sprout in the pods every once in a while.
Rain, heavy dew and high humidity are probably all factors. Late in the fall, I find quite a few sprouts in unharvested pods that have been rained on, during long stretches of cool, damp weather when the garden is getting more shade. I've seen tiny roots hanging down out of the pods a half-inch. I think this might be a different phenomenon than a seed which begins to grow in an unopened pod.
There is no rain or dew all summer long here, and I still find the occasional seed sprouting in fresh pods. My guess would be that these are seeds which somehow developed without a sprouting inhibitor. This could be one of those random "mutations" which occurs as a survival and adaptative mechanism—if all seeds sprouted on the same strict schedule, a slight change in weather or climate could eradicate a species.
The last two sprouters I found were from a green, thick-walled, freshly split pod on Spartan Warrior. The pollen parent was Damien Charman. I planted them about 3 weeks ago—one is about 1.25 inches tall, the other has just the tip of a leaf showing from the bend of the hypocotyl, and appears to be "confused", as if it really would have appreciated some stratification. I'll keep track of them, to see what kind of growth habit they have.
There were a couple of other pod-sprouters this season, but they didn't survive. Unfortunately, they came at a time when the weather was warm and dry, and I was too busy to give them the attention they needed.
Ken