I know why it happened.
theory time right now GO!:
well zetsu came from underground to get inside kaakshi's body.
cuz of him and obito's sharingan. Madara cast a spell on obito's sharingan.
it made obito see the future of rin getting killed as well as kakashi's tears.
obito said he thought it was rain.
zetsu told him not to worry about images he sees, cuz zetsu knew what was happening.
madara tried ot make it look llike they wanted the sharingan.of kakashi through mist ninja.
part 1 of the theory, do you like