If you received BTC from your dad or brother who also purchased BTC from coinbase then they may think that you are in control of their accounts and this may be behind the question of asking about other coinbase accounts that you have. If this was their thought process, then they might be afraid that you were using multiple accounts to get around their daily limits, which could potentially expose them to additional credit risk.......this is of course speculation.
Yeah that makes sense but honestly we are not talking about thousands of coins or anything like that, we are talking low small fry numbers. If they truly yanked by account for 10 or 20 coin transactions here and there I would be very surprised. Looks like their policy over there is
guilty unless proven innocent, or rather
guilty guilty guilty despite evidence proving innocence. I'm gonna use Bitstamp from now on