Help:tomcat 6.0.18 showdown automatically!

[red phoenix]

I use tomcat 6.0.18 deploy struts2.1.8.1+hibernate3+spring3.0,I find a
puzzled question,I find tomcat will be shutdown automatically after about 10
minutes,I look up files under logs directory of tomcat,I find
localhost.2010-05-09.log,it looks follows:
2010-5-9 22:36:12 org.apache.catalina.core.ApplicationContext log
Info: Initializing Spring root WebApplicationContext
2010-5-9 22:45:12 org.apache.catalina.core.ApplicationContext log
Info: Initializing Spring root WebApplicationContext

The I look server.xml,
...
<Host name="localhost"  appBase="webapps"
            unpackWARs="true" autoDeploy="true"
            xmlValidation="false" xmlNamespaceAware="false">
<Context path="/MyApp" docBase="MyApp" reloadable="true"
crossContext="true"></Context>
...
I look up reason,it looks spring will reboot webapplication,so I delete
following statement:
<Context path="/MyApp" docBase="MyApp" reloadable="true"
crossContext="true"></Context>

Then I start Tomcat,this time tomcat don't shutdown after 10 minutes,but
about 30 minutes,tomcat6 also shutdown automatically! this time I look at
localhost.2010-05-09.log,I don't find following information twice:
Info: Initializing Spring root WebApplicationContext

I am puzzled with it!!! Why tomcat will be shutdown automatically? What
reason raise tomcat shutdown  automatically? How to solve it?

Any idea will be appreciated!
Best regards,
phoenix