Hi ND,
There are a couple things that can cause this. For example, if you a user has 1,000 messages in a box, then just deletes them all, it might consume the rest of your RAM, and cause tomcat to crash. That's not a bug, it just requires more memory.
That, however, is less likely. In order to solve this, we'll need the /opt/zimbra/log/mailbox.log and /opt/zimbra/tomcat/logs/catalina.out
Keep in mind that cat.out gets rewritten each time you start the Zimbra server, so you'll need to preserve the "evidence"
It would also be useful if you updated your profile to reflect which version of Zimbra and OS you're running, for future reference.
Good luck,
john