Pooled logins/logouts causing overloaded server

I have an ASP.net web application with a large amount of user v开发者_运维技巧isits. I have used NHibernate as my ORM (with SQL Server 2005 SP3), and it has worked fine. However, it has been about two months now that my dedicated server has reached 100% CPU usage. I found that 90% of this usage is from SQLServr.exe, so I used the NHibernate profiler to find the problem.

I see this in my profiler trace result:

Audit Login
(Some Query Execution)
Audit Logout

When I look at the "EventSubClass" column, and all login(s) and logout(s) were pooled!

What is the problem with these multiple logins and logouts?