There's no guarantee this isn't what's been done on most low volume exchange especially the scammy ones that refuse your withdrawal due to them not having enough bitcoin or any other coin to process this volumes coming in. The big exchange though would hardly try this since they know they're been closely monitored and they can easily top on their reverse since they make a lot of money from trading, listing fees and all other avenue they used to generate income.

Before now I always thought each time an exchange goes on maintenance is due to them not having enough funds for processing withdrawal, exchange likes mercatox was known for always suspending withdrawal for trending coins and it was becoming obvious they lacked the volume. Exchange like yobit and Livecoin could easily be guilty of this practice and I won't be surprised because they lack the volume and their mode of operation are shitty.