I have myself faced this problem twice. Once in blockchain and once in Coinbase. Actual reason behind this is that confirmation is taking long as mainly you are sending bitcoins to a different wallet provider. (E.g- bitcoin to Coinbase).
This usually takes some good amount of time in some cases. In some cases it happens in a snap. Whereas others, it can take up to a day or few days too. There's no actual reason told by blockchain about it. But, it's just for adding more security for transactions to other provider's wallet.