From your screenshot, it looks like you are using Change Packages. You can see who dropped the member from the Change Package details - the owner of the change package will be the user who dropped the member.
To find the right change package, you use Find Change Packages. For example:
You can also see the change package from the Project Differences view:
If there's a long list of change packages, then finding the right one is more difficult using Project Differences.
If you are not using change packages or the user did not use a change package to drop the member, you can look at the audit log, if you are logging drop member operations. If there's no audit logging or change package, then the only option is to look directly in the database. In that situation, I'd recommend contacting PTC Support to assist.