Note that I didn't say 'update', I said 'check the integrity'
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It might be a bit aggressive on that front, but on the other hand, it does have to do *something* to check that the index of a book points to the right book, or that that book hasn't been updated from under its foot, or that there's not some new annotations that snuck in... And same thing when it checks for new contents, it also needs to check/purge the database to prune index pointing to deleted/modified content, and then check/index said new contents. (I'm not privy on the details of the implementation, and it might very well not be the most battery-friendly choice, but it does try to ensure accuracy & consistency, which is what they aimed for, I think).
I don't know the extent of what it's doing exactly when you wake it up, but it is doing something, especially if you wake it on the Home screen, as the logs and a top output can attest to
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