Wikipedia also says the same thing, both in English and in Japanese, and in both languages uses this report[1] as the source. This in turn sources a medical journal and a report by a mountaineering club. I don't know Japanese and can't really check whether these are reliable sources that actually exist. Essentially what this report says is that after WWII, particularly in the late 50s and the 60s, there were lots of climbers and unsafe routes, causing a large number of fatalaties. Afterwards there's a sharp decrease, though still pretty significant numbers.

[1]: https://soka-yamanokai.com/study/study18.html