Some times it's triggered by conditional access policy wrt device location. We have devices set to up to 30 day session if the device stays on our company network, if you take it off site or even hotspot it onto your phone you gotta sign in or provide an MFA code
This isn't 100% true for every case. I'm able to stay signed in to multiple accounts and switch through them no problem. Doesn't work for my boss on the same accounts. (Firefox finally wins one against chrome)