I am not sure exactly what acts as a signal to trigger them to drop from the nests. I am pretty sure it is weather dependent but whether it is something simple, like reaching a minimum temperature, or whether they sense a range of parameters I am not sure. I know that ant swarming is highly temperature critical and sometimes they will start to exit from the nest as flying ants and then retreat back in.
While the nests tend to be built where they will catch the most sun in winter it is not hard to imagine that each nest would reach some critical temperature on different days depending on their exact location. If you drive around on a motorbike at this time of year you certainly notice that there are warm and cold pockets of air.
Also the temperature doesn't steadily increase from day to day. You might get a warm day followed by several colder days. This could also mean that one nest, perhaps in a particularly favourable location, reached its critical temperature one day while another nest in a slightly less favourable location didn't. It might then be many days before the temperature overall got back up to the same level or slightly higher to allow the second nest to drop.