This looping occurs when a user is moving from a SharePoint farm in one geo to another. The problem is only occurring with IE browsers, and only when the FedAuth cookie is present on the request. The looping is occurring between /_trust/default.aspx and /wsfeddispatcher.
Single Sign On is generating an assertion as expected with every visit to wsfeddispatcher, but SharePoint is redirecting the user back to wsfeddispatcher instead of authenticating and redirecting to the application.
Contact Microsoft for a solution if you encounter this problem.