Session will be interfered by another application's thread

Hi, I have a problem about session and application. The situation describes as follows.

I have two web site A and B. They have different AppDomain.

First of all, I use session to record some info. in A.

While I invoke the application method "Applicaion.RemoveAll()" inside a thread, the session of web site A will be expired.

So web site A could not access to any value of session.

But if web site B do not use thread to invoke this thread, the session of web site B would not be affected. I dont know why it cause session expire.

So far as I know, since A and B is different AppDomain, why session would be interfered by another application using the thread.

Thanks for any kindly answer.

Jim.