Did you added some component to the assembly to which it started happening as there is a possibility that some component itself or its entity (sketch/plane etc.) might be lying far from assembly origin and due to which when you're doing zoom to fit, it the extending all the way to that component or entity. Try suppressing the last component in the feature tree and zoom to fit. Repeat this along the way up i.e. suppress a component and zoom to fit. You should be able to find the culprit.
If this doesn't work then pack and go the assembly as zip file and attach here so that one can find the issue.
Please use "advanced editor" in case you want to attach any files.Click on reply and then click "use advanced editor".