He kinda jumps back and forth. There was about a year of ~2-month releases after the big DF2010 release. Then this cycle started and things seem like they kinda got away on him.
I would personally chalk the length of this cycle up to the creature drive. Toady has stated he felt compelled to do an extra good job on these creatures. This meant that he was looking at ways to make the creatures unique, so he started the interaction system. Then he added squids and so material breaths got merged into the interaction system, so there were interaction attacks. Then he noticed that he had just made all these cool catacombs and stuff, but there was nothing in them, because undead were super lame, so he added undead to the interaction system, and if undead are created through an interaction then why not add necromancers? At this point Toady pretty much just decided that, since he was already on a roll, he may as well flesh out the night creatures completely. That brings us to today, after werewolves and vampires and mummies and catacomb-dwellers have all been loaded in.
(note: this is not speculation; this is all just what I remember from devlog entries)
So yeah. Cities (what was actually on the dev page for this release) were mostly done around July or so, and since then has been night creatures and interactions, all spurred by the new animals. What is somewhat funny about all this is that the animal sponsorship drive was envisioned (as I understand it) as "an easy way to encourage donation without slowing down development significantly" since, had Toady just made RAW entries for the described creatures, it would have taken no time at all. What ended up happening though is that he wanted to "do [each creature] justice" because people had paid for them.
Anyway, there's your year-long release explained.