SSIS package blocks itself if uses TRUNCATE
This is due to blocking caused by the delete task accessing the metadata from the database during the Pre-Execution phase.

The locks can be found using sys.dm_tran_locks and it will have -2 value as OP founded. More details could have been found out using Profiler.

The blocking can be resolved by changing the ValidateExternalMetadata property to FALSE or as OP mentioned using a DELETE statement.

Reference: KB2253391