From my observations of activity on my account, it looks like iStock are purging (i.e. deleting) all their storage of rejected files (why they originally decided to keep them is another question).
I am guessing that this may be a direct result of their new ingestion policy (i.e. no upload limits, and accept any old cr@p), so perhaps they are having to free up some storage.
Could also be why the rate of ingestion (i.e. sloooow inspections) has slowed down.
