Bitcoin transaction fees move corresponding to chain activity and miner reward maximization. A side effect of this is that transactions with lower fees either get stuck in the unconfirmed pool.
Coinbase has begun re-transmitting such transactions with the requisite fee so that they get confirmed.The resent transaction essentially pays the fee difference for the earlier transaction. Coinbase does this without their customer being in the loop, for better UX. This process is detailed in the following figure.
Source : Coinbase Blog