How to discover the reason of ASP.NET application restart

Have you looked at the logging suggestions from "Logging ASP.NET Application Shutdown Events "? That would be my suggestion for a way to get more information about what is happening.

Adding or removing folders inside the applicaton folder will cause the app pool to recycle (the most likely reason in your case), along with many other things. Here's an interesting article:

http://blogs.msdn.com/johan/archive/2007/05/16/common-reasons-why-your-application-pool-may-unexpectedly-recycle.aspx

on the same site is some good hints on how to monitor the restarts which is what you were actually asking. The choices are web.config and EventViewer or Performance Monitor.

http://blogs.msdn.com/johan/archive/2008/02/18/monitoring-application-pool-and-application-restarts.aspx