There’s the General Collapse theory: He makes some unrecoverable gaffe, or has a meltdown in a debate that he can’t put behind him.
There’s the Consolidated Challenge theory: Progressive voters unite behind one candidate immediately before (or after) Iowa; and then wage trench warfare, delegate-by-delegate, fighting Biden all the way to Milwaukee.
There’s the Hand of God theory: Some event slingshots another candidate past him. This would have to be something major, like a Michelle Obama endorsement or a financial panic.
Any of these things could happen. Electoral environments change frequently. Remember the 2004 Democratic primary and the 2008 Republican primary, when the presumed front-runners tanked, then came back from the dead to run the tables? Or the 2012 Republican primary, when just about every declared candidate—even Herman Cain—had a moment at the head of the pack?