It appears that the February start of production is no accident since Darabont told the Comic Book Movie site that he is very interested in shooting some episodes in winter: “It would be great not just to get out of the heat, but to present a different idea to the audience visually and tonally by having it be winter. There’s some really cool stuff that Kirkman did, where they find the one zombie that’s frozen to the ground. I’d never seen that before and that’s really cool.”
There’s another reason for AMC to get cracking on a second season. With only six episodes in the first season, there will be a pretty long gap before the second season begins, which is not necessarily a good thing for a hot series. By starting pre-production now and actual production in February, the network should gain the flexibility to air season 2 episodes next summer or even in the spring.