Richardson may also be taking some from the stealth member of the top tier, "undecided".
I tend to think it's "other" ("other" and "no clue" were where the drops in this month's DKos poll happened), and that specifically it's the current unofficial fourth top-tier candidate -- Al Gore.
I guess if there are some Gore supporters that liked both Sustainable Energy and NAFTA style bogus free trade agreements, they would be attracted to Richardson.