Amee realized the unfair treatment toward Iris had started taking toll on her and with Sofia being here, it only escalated things further to the worst case possible, as you could see the contrast between the way they treated their own luna and an outsider who claimed to be carrying the Alpha's firstborn.
Kaz didn't even wait for Cane's instructions or confirmation before he rallied people behind him to hail Sofia as the new mistress. Some of them were desperate to find another 'Luna figure' aside from Iris, because they couldn't stand her.
Not because she was not good enough, but because every time they saw her, subconsciously they would be reminded of Gerald. It made things hard for them, as well as Iris.