In the US we would have to look at a fourth possibility in the opening list: that Rudd went out and "bought" (or bought into) a few "members of corporate Australia."

It is possible that a candidate might withdraw due to "lack of support" and return to candidacy after surveying the support he/she has more accurately - and maybe getting a few more firm "commitments" from people who couldn't be counted on at the earlier time.

Having no knowledge of Aussie politics ¹[other than what I read in the US press] I'm only suggesting a theoretically possible explanation, of course.

¹ text in "[" "]" is redundant with "having no knowledge."

John