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.

Chromium Extensions official support forum

I try to summarise common knowledge. I experienced the "User Data Privacy" and "Spam & Placement in the Store" rejections.

"User Data Privacy"

  1. Check if you have a Privacy Policy link. It's required now. You could use some template (template I use - https://docs.google.com/document/d/e/2PACX-1vRo7pRGkrPkeucDrEMpUVuhPeiPEhxy9h55kQTXQuXpfdWSAhHdFqjohyl68d_Aa2AYPmkKoDq1ZNWn/pub)
  2. Check if you have permissions that you don't use.
  3. Consider using optional permissions (https://developer.chrome.com/extensions/permissions)
  4. Consider using activeTab https://developer.chrome.com/extensions/activeTab instead of <all_urls>
  5. 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"

  1. Too spammy description. Remove content that is not necessary for your description.
  2. 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.
  3. If you use black hat tactics (fake installs and review, etc). Ask me if you want to know more.