The same guys sponsoring Hillary....are probably sponsoring Bernie. You want a marginal secondary candidate....so he makes perfect sense. If he ends up winning in Iowa....watch for him to suddenly fire his staff to ensure he starts coming in second.
Shades of Perot getting out of the race when it looked like he was about to win, then getting back in just in time to take enough votes away from Bush to get the first Clinton elected.
Yep, he is only there to enable Hillary to stay in the "center".