Answer by tbxi for AはB emphasizing B, rather than A

が is not always emphasis, it is also used as a subject marker in the subordinate clause, and things like passive form.

(私が)持つ200円を私が落とした。 - It was me who dropped 200 JPY (derp!)
私が持つ200円を落とした。 - Dropped the 200 JPY carried by me.
私が持つ200円が落ちた。 - It was the 200 JPY I had that made itself mobile.