No way to tell how or why. Postmaster should only delete stuff if it gets full and you continue to pick items up. The oldest items get deleted to make room for newer items. Regardless of how or why, Bungie does not restore lost or deleted items and characters so the only way to get them back is to complete activities that reward them. The only suggestion I can give is to try and keep your Postmaster mostly empty at all times. It should never be used for storage. Alternatively you can use third party apps/sites to set up a farming mode where they will continually keep your Postmaster cleaned out to avoid items getting auto-deleted from a Postmaster getting full.