You don't think it might be because he's going to release more of his tax records, and he wants to be prepared to release his running mate's in corresponding length without any surprises?
That is a possibility but runs counter to everything Romney has said so far. You are right though, best to have several years of returns to avoid any nasty surprises.
The only reason the tax returns are a big deal is because Romney is acting like he wants to hide them when there probably is no reason for him to. But his approach causes suspicion when many people already don't know much about him.