When you press the back button at Detail
Activity, it gets removed from the back stack, now
when you open it again a new instance of that
Activity gets created, the one you had been
previously using no longer exists. AsyncTasks are
tied to the UIThread Object that creates it, if
that Object gets killed then so does the
BestApproach to Download something is to use
the DownloadManager class in Android.
Hope i helped.