Responses
The definition of Tragedy (at least to those who major in theater and literature) is an event that has a steep downfall that could have been prevented. So, many things that are listed as 'tragedies' nowadays, are not truly tragedies, but just sad stories, and other stories that aren't marked as tragedies but people claim they should be, are just people who complain while being misinformed. .........sorry, I went off on a tangent. you are most likely correct in your assumption that these people marked it tragedy because of an occurrence in the past, or just a sad event.
This is the only website I've seen where this manga is listed under 'tragedy'. Tragedy doesn't mean that it has to have a sad ending, it sometimes pertains to a character's past. Or the tag could be a mistake (which I hope is the case)