Extensions are set to auto update by default, and it's not obvious at first how to disable that. It's also disallowed to install an extension you've built from source on most release builds, without messing with a hex editor. So essentially, any extension you install is liable to be come a "random" extension, if for example, the author sells out, or something like the attacks on NPM were to happen.
And sometimes the unscrupulous third party decides to build a browser and take over the market via forcing hardware vendors to bundle it with their OS. Like Google.