In production all classes last modified date has been changed to a same date. We have 6MB of apex code in production. We haven't done such huge deployment. We do it by changeset and sometimes we use eclipse. Is there any possibility by which all classes would be changed?
Yes. If you are using eclipse then you can modify apex class in production.
I think anyone saved your workspace to server(i.e. Deploy to server). not sure only guessing this can be a reason.
Check login history may be you will get something.