How to run a long query from a trigger without upset
I would recommend you to use "SQL Server Service Broker". It allow you to run queries asynchronously not only in triggers.

Here is the link to documentation: https://msdn.microsoft.com/en-us/library/bb522893.aspx

Here is a link to a tutorial that works through how to set this up: http://www.sqlservercentral.com/articles/Service+Broker/73398/