Is throwing birthday parties for veteran employees part of Japanese culture?

Hello, I am an expat who works at a Japanese game studio.

I have worked here for a few years by now and recently stumbled across this reoccurring event that I found perplexing.

Every year there is a birthday party for this veteran employee (afaik they have been in the industry for 30+ years). The party is arranged by the kouhai of the person in question but pretty much all of the participants are fellow coworkers at the same company.

I have never seen this done for anyone else (there are other people with comparably lengthy careers at this company), which makes me wonder if this is cultural or just something the kouhai takes on themselves. (My guess would be the latter but idk)

If it is a cultural thing, I would love to know more about the reasoning and nuance behind this.

Thank you for reading!