According to Metro Application Lifetime sample and related document, it description about Metro app about suspend/resume.
Here is my summary for this:
When leave app about 5 sec or trigger another app about 5 sec, the “suspend” event will called.
“Suspends”/”Resume” event will trigger automatically when you not attach debugger on release build. See “Star without Debugging” on “debug” tab.
If you using debugger, you only trigger “suspend” manually.
Sleep/Hibernate will trigger “Suspend” if your app is release version without debugger attach.