Why would one endodontist go back in and redo a root canal his partner started>i saw films and it was a big redo.?

See prior answers ++: One of three reasons: (1) first dentist got into a problem that became beyond his expertise, and referred to his partner, who has more experience and expertise about the problem at hand (most likely; give them the benefit of the doubt). (2) this is a routine way of approaching root canal problems in that practice, and perhaps it wasn't and redo, but next stage. (3) $ (less likely.

Healing varies: Sometimes after even great looking root canals healing is incomplete or the body doesn't react as expected. The most conservative thing to do is to redo the root canal, perhaps with a different healing result. Root canals are tiny 3d structures and cleaning and filling them is an inexact science and art.