+1 that this is almost certainly a terrible idea.

If the trigger has a lock on the table and you have a source(/destination?) component reading the same table it could be the metadata check that fails to achieve the required lock. In the pre-execute phase (I think) of a DFT, SSIS will verify the metadata of sources and I believe this requires a schema lock.

You can set ValidateExternalMetadata to false on any source components to avoid this check.

Are you using transactions?

Maybe the package executes fine through a job because the trigger just initiates the job and then completes and releases the lock, whereas if the package is called directly the trigger waits for control to be returned?

Just some thoughts.

HTH,

Sam