... Fall spawns do not exist. Crappie are a once a year spawning fish. Some find egg sacs in the fish they catch, in the Fall, and think they're in the process of a second spawn. They are not. They are, in fact, in the beginning stages of egg development, that will take through the Winter months & the rewarming of the shallows to begin the process. Crappie young (fry) cannot overwinter, or survive in the colder waters.
Crappie may be caught trolling the flats, in the Fall, simply because that's where the baitfish are ... as they migrate into the larger creeks & bays, seeking warmer (shallower) water to feed in. Thermoclines may have built up, with the warming of the upper portion of the water column. This can account for the baitfish & predator fish being in the upper layers of the water column. The water may not be as comfortable, temperature wise, but it retains more oxygen than the cooler water beneath it. The flats can be productive when they're within the depth of the upper reaches of the water column, above the thermocline. Generally speaking, the shorter days & cooling water temps are a sign to the fish, that Winter approaches.
Some interesting reading :
BNM Fishing
Fall - The Other Crappie Season
BNM Fishing
BNM Fishing
... cp