I think the Rock Trolls took her memories to protect her from something in her past like Anna. Maybe Elsa froze something or someone else and SQ ended up taking the blame to protect Elsa. They took the memory to prevent it from haunting her the rest of her life.
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Nothing is impossible on OUAT. Everything has a way of repeating itself. Keeper of the Keeper tread: https://oncepodcast.com/forums/topic/official-keeper-thread-3/#post-217924