The White Mage had used her magic, whiteout, and brainwashed nearly all of the Fairy Tail Guild. One of those brainwashed was Mirajane. The White Mage had ordered all of the brainwashed guild to go and wreak havoc

Mirajane: was currently trashing some of the homes in Magnolia when Erza found her Erza, you came to play with me~. She smiles

Erza: Mira, you're not yourself. You're under a spell.

Mira: The White Mage simply opened my eyes, Erza. Her hands began to glow with magic Come on, let's play like how we used to when we were kids. She says with a smile, acting like her young rowdy and competitive self