Most likely it's due to high trading volume and Coinbase's servers just can't handle the traffic. We all know that crypto currency is highly volatile however coinbase does not have an option of setting a stop out price. So lets say for example Ethereum drops 30% you wanted to sell when you saw it drop more then 10% and the server is down and no one can access their accounts. Then what, you just going to have to eat that 20% loss? Imagine if Apple stock dropped 30% and you get locked out of your Scottrade account and you lost 30% because of technical difficulties, wouldn't Scottrade get sued by everyone?