Is there a way to automate the process to unlike all Facebook Pages at once?
There is no official way to unlike all the liked pages at once. You have to unlike one by one. For that type "Pages I like" in search bar at the top on right, it will list all the liked pages and you can unlike one by one.
To unlike all the liked pages at once you can use third party tool.
Facebook Social Toolkit
It's an extension for Google Chrome. Download and install Facebook Social Toolkit form chrome web store. Log in into your Facebook account. Start Facebook Social Toolkit by clicking on Facebook social toolkit icon. Click on unlike all pages button and then click on OK button.