They may be acting on behalf of a fund manager who bought the...

They may be acting on behalf of a fund manager who bought the stock and settled. In a cash account the custodian cannot lend out customer shares as he is only providing settlement services and safe custody to client for his investment in the company stock. He can lend out shares only if the custody agreement explicitly provides that he can lend out customer shares. Normally fund managers do not agree to this clause.

When you open a margin account with a broker they are free to lend out your shares even if you bought them for 100% cash as they have given you a credit line against the shares and hold indirect title to the shares.