Chrome Webstore review process in tough and long today. Your extension could be rejected while you're updating a version to fix some small bug.
You will receive an e-mail "Chrome Web Store: Removal notification for <your extension>". In the e-mail, you will be referred to the policy that you violate. All policies are specified here.
Chrome Web Store developer support provides you a vague reason for rejection.
Also, you can check support forums for up-to-date review process information.
I try to summarise common knowledge. I experienced the "User Data Privacy" and "Spam & Placement in the Store" rejections.
"User Data Privacy"
- Check if you have permissions that you don't use.
- Consider using optional permissions (https://developer.chrome.com/extensions/permissions)
- Consider using activeTab https://developer.chrome.com/extensions/activeTab instead of <all_urls>
- If everything seems right, just reply to an e-mail with justification for each permission and submit again. It worked for me once
"Spam & Placement in the Store"
- Too spammy description. Remove content that is not necessary for your description.
- Use of brands e.g. Facebook, Youtube in your description. I don't know exactly how it works, but brands trigger this kind of rejection.
- If you use black hat tactics (fake installs and review, etc). Ask me if you want to know more.