Your explanation is a little short on details.
The log file in $SPLUNK_HOME/var/log/splunk/splunkd.log will probably have some hints as to the source of the shutdown. There may also be a crash file in that directory as well. If there is a crash file, it will probably have a hint as to the nature of the problem. But this hint may only be useful to Splunk support or Engineering.
Make sure your AIX system has the bos.adt.debug package installed - this is needed for the crash files to have full stack trace data.
