Yoshihiro Togashi's classic series Yu Yu Hakusho is mostly known these days for its animated adaptation, but the original manga had a very different, very sudden ending that came about for some truly tragic reasons.
Originally, Yu Yu Hakusho ran from 1990 to 1994 in Shonen Jump, and was Togashi's first big hit manga. The story follows Yusuke Urameshi, a boy who dies and is brought back to life to act as an agent for an afterlife bureaucracy, sent to investigate supernatural occurrences on Earth. However, this premise is quickly cast aside as the threat level of enemies increases radically, turning the series into a particularly dark take on the shonen martial arts and action genre. It's well known for its use of tournament arcs, with three separate tournaments taking up the bulk of the series. The manga had a very controversial ending, which seemed to come out of nowhere and implied that the heroes had merely been pawns for King Enma of the spirit world. Many fans were understandably upset, and the anime went out of its way to establish a different ending.
When a series ends on such a strange note, speculation runs rampant throughout the fandom, as it can be legitimately hard to understand why a creator would seemingly turn their back on their own story. Details slowly emerged, from Togashi himself as well as others, which eventually shed some light on his reasoning. As it turns out, Togashi originally wanted to end Yu Yu Hakusho at the end of the Chapter Black/Sensui arc, a full year prior to where it eventually concluded, but was pressured to keep the series going due to its popularity. In a doujinshi (unofficial manga) that Togashi released called Yoshirin de Pon!, he further explained his reasoning, saying that while he still cared for the work, the stress and how it dominated his life simply became too much, and so he ended the series with such finality that no one could continue it. At the height of its run, Togashi was allowing himself as little as five hours of sleep a night and it began to take a toll on his health, even at his young age at the time.
Togashi's story is sadly common in the manga industry, particularly for newcomers. Since the field is so competitive, publishers tend to have the upper hand in negotiations, and push difficult (or sometimes impossible) deadlines that force the most dedicated mangaka to sacrifice their health or face cancellation. For example, the creator of Bleach Tite Kubo had a similar breakdown that led to the end of his first series, Zombiepowder. Togashi himself still has health issues to this day, but his popularity gave him a better bargaining position for his next big series, Hunter x Hunter, where he was allowed to publish on his own schedule. He also occasionally receives assistance from his wife, the creator of Sailor Moon Naoko Takeuchi. In some ways, it's similar to the Hollywood film industry, as eager artists with big dreams are taken advantage of by massive publishers.
Togashi said that his reasons for ending Yu Yu Hakusho so abruptly was "selfish," but in fact, all he was asking for was a healthy work/life balance, as everyone should strive towards. Especially in the manga industry, it can be tough to keep yourself artistically fulfilled, keep the fans happy, the publisher happy, and still have a life beyond work. For their part, fans can encourage change in the industry, and be more understanding of hiatuses. Hopefully, mangaka like Togashi will be able to work at a more sustainable pace, and an ending like Yu Yu Hakusho's won't happen again.