Isilon NDMP backup related snapshot management/deletion

We are doing NDMP-DSA backups for EMC Isilon using EMC networker to the Data Domain.

We use the following variables under the "Application Information" section of Isilon client properties in Networker:-

BACKUP_MODE=SNAPSHOT

We are seeing lot of snapshots left behind by the NDMP backup(some get deleted every night and some left behind).

Is that normal and if not then what should I check?