What are the possible causes for an SSIS package execution having an "ended unexpectedly" status

I have a SQL Agent job that runs an SSIS 2012 package every 30 seconds to export a small amount of data to a CSV file. There is often nothing to export, but when there is, the business wants it done ASAP. This has been running fine for months without error, but over the weekend it had 3 failures: Start Time: 7/25/2015, 11:43:13 PM, End Time: 7/25/2015 11:44:36 PM, Status: Ended Unexpectedly Start Time: 7/25/2015, 11:52:05 PM, End Time: 7/25/2015 11:53:13 PM, Status: Ended Unexpectedly Start Time: 7/26/2015, 5:01:31 PM, End Time: 7/25/2015 5:01:31 PM, Status: Failed In each case, there were no records in internal.operation_messages for the operation_id. The All Executions report also showed no Messages and no entries under Execution Overview. Normal durations for this package are in the 4 - 26 second range. So the **Ended Unexpectedly** executions had unusually long durations. The **Failure** execution had a duration of zero. There are no maintenance jobs of any kind running at the time of these failures. I found this in the Application Event Log that correlates to the time of the Failed execution: > The operation 414582 (GUID:82f992bf-ed2a-4d82-b25d-92a9d3052cd9) > terminated because the AppDomain 488 is unloaded. Try restarting the > execution. I don't know what this means. I found instances of this message that correlate to the "ended unexpectedly" executions: > The SSIS Execution Process could not write to the IS catalog: > MyInstance:SSISDB Error details: Timeout expired. The timeout period > elapsed prior to completion of the operation or the server is not > responding What are some possible causes for these?