The resurrection has a considerable role with Rennala. She was essentially defeated by the Tarnished at the end of the first phase, so it's understandable why her ability was triggered. It also happens after Sellen's questline which was basically she killing everyone in Raya Lucaria, so it's a pretty simple interpretation. Also, could know why Ranni protecting her mother has something to do with illusions? I mean, I didn't negated that tho, but as I said it's mostly she giving Rennala more power than she had once. Of course, after defeating her she will lose the amp, because that spell for protection was dispelled, and with that she will return to her normal state.