For some discussion, see https://languagelog.ldc.upenn.edu/nll/?p=3828

If you can shed light on something, why can't you shed a couple doubts on it too?

Because that would be ridding yourself of doubt, not gaining new doubts.

If you "shed light" are you not casting light or exposing to light the subject?

So in this context you would gaining new doubt.

I always assumed this came from shod as in a horse shod with shoes.