In share-point permission settings(both in Document and Library level) is it possible to track who have removed the permissions of others.
If you enable the audit logging on your site collection level then you can track it.
You can use the audit feature of Microsoft SharePoint Server 2010 to track which users have taken what actions on the sites, content types, lists, libraries, list items, and library files of site collections
Another way, required hard work, you can check the IIS logs and look for the permission page url and see how access it.
I see that you use SharePoint online. So It's possible, but you don't set it at library level. Instead it should be set at site collection level. Just go to Site Collection Administration - > Site collection audit settings and select interesting events for you. After that you can generate report with Site Collection Administration - > Audit log reports