According to its wiki page, tied up is for physical restraint in a purely comedic or otherwise non-sexual context, in contrast to bondage, which is sexual. But almost the entire front page of tied up is sexual. Oops!
There also seem to be some obviously non-sexual examples (Mystia in post #1325590) that are tagged as bondage, but there seem to be fewer of them. This could of course be because there's simply more content of sexual bondage than non-sexual binding on Danbooru.
For cleanup, I think a good start might be to remove tied up from all explicit and questionable images tagged bondage, and then start examining images tagged bondage to look for ones that should be tagged tied up instead. Of course, we may want to rename some tags before trying to fix the problem, since these problems normally stem from confusion or ambiguous names. What to change the names to will be a tricky question, though.
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