Safeguard (goat drench) is given once, then again after 10 days to kill any hatching larva that escaped the first treatment. This is done to break the life cycle of the parasites. Otherwise, the hatching larva in the gut can just reinfect the bird and your first treatment was useless.
I like the idea of worming in late fall when egg production is lower so I don't waste as many eggs. Also, when the soil is cold and dry I think that reinfection is less likely to happen as quickly. I don't know if there is anything to that, it's just my theory. Also, by the time winter comes, they already fat and healthy.