It is a bug and was reported 2 months ago here. This happens when someone accepts your answer and subsequently adds a comment to this answer. So you should usually find a comment close to this acceptance in your Notifications. This would then be the question that was accepted.
If you are following the tag, then you can see it in the Activities too that an answer was accepted, but here you don't see from whom this answer was, I can usually identify my answers by the words in the preview. (You may also see another entry which says that the user performed a rejection of the right answer)
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