Because IIS restart the pool (including sessions) each x minutes with no activity, configured by the idle timeout in the settings of the pool itself, in the case if user set 90 minutes of session in the app, if there is no activity, IIS can restart the pool before this 90 minutes ends. Example:
Session is 60 minutes configures in app.config
after 30 minutes of idle, the pool recycle itself
Only one person use the app
1.00pm: user connects, he navigates during 10 minutes and then do nothing on the page (for instacne fill a very large form without submitting, and without ajax calls. At this point, user has session "open" untill 1.00am + 10 minutes + 60 minutes configured = 2.10 am
At 1.50 am, he tried to press submit button but it doesn't work because the app pool was recycling at 1.40 (1.10 + 30 min of idle) so user lost session.
If this is possible that user is anole on the app, idle time must be the same of greater than session time.