Modern browsers run each tab and extension in its own process so that one failure does not take down everything. When the whole browser closes anyway, something shared has failed: the profile data, an extension hooked into every page, or the memory available to the system.
Extensions are the leading cause. They run with wide access to the pages you visit, and an update to a single extension can start crashing tabs on sites that worked yesterday. Testing in a private window, where extensions are usually disabled, is the fastest way to confirm this.
Profile corruption is the second cause. Your browser keeps settings, history and cached files in a profile folder, and damaged files there produce crashes that follow you between sites. Clearing the cache addresses the mild version; creating a fresh profile addresses the rest.
Memory is the third. Browsers are happy to use everything available, and a session with a hundred tabs on a machine that is also running other heavy apps will eventually fail. Crashes that occur only late in a long session usually point here.