The Black Blood is what caused it. The little demon (basically the personification of the black blood) inside of Soul pulls Maka's conciousness inside of Soul in an attempt to claim them both. When Soul finds Maka inside his mind, they discuss using the Black Blood's power in order to help understand Crona (who they are currently fighting). They agree to do it and Soul promises to pull Maka out before she completely loses her mind to the blood, which he does.
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