With Haachama, it's going to simply be stress. She should be finishing up university this year, we don't know what she's studying and spent the first couple of years attempting to maintain an active, full time Job that required hours of performance. While not talked about much, the standard Japanese diet is actually pretty bad minus the heavy fish consumption, and if she got sick it could have easily taken a long time to recover. Which is a problem when you're a full time student and a Hololive member.A mental breakdown is probably a better answer. Japs have never been good with mental illness and given her workload I think its more likely that the stress finally broke her.
This is how you get Hikkomoris, realistically. It's also likely her family stepped in to help and/or made some decisions for her.