Lots of things. But nobody can guess until you've provided more details.
How big is your index?
How much memory do you give the JVM?
what were you doing when the error occurred?
Are you sorting over many unique terms?
Are you simultaneously updating your index?
etc.
Perhaps reviewing this would be
After a much closer look at the logs, I found these errors:
Looks like the errors started from the 3rd day after my server was running.
After the initial Java heap space error Jul 15, 2010 9:46:34 AM
Solr did run for a certain time till Jul 15, 2010 8:13:33 PM when it
started failing due to the
