I like Opti's answers here but you could always tell them you bought and immediately sold something, like a car. Some kind of private transaction.
Seems like the amounts are too high for the usual, computer components ploy.
Whatever you do, don't tell them you don't like to be questioned about account patterns.
They not only will question, they will investigate and, like Opti said, may look for statements, or more.
A private, personal transaction will less likely warrant further inquiry.
Tell them you might do it again in the future but don't plan on making it a regular habit, especially if they don't want you too.
As far as other future use, tell them you are getting into trading but may want to store coins for a bit, or that rare potential for private transactions, tell them sometimes you get deals on cars or something with some known crypto traders, or guys that like to deal in cryptos. These answers can be innocent and the thought of the storage and trading will get them looking away.
Then, just chill on moving coins in and out, they really frown on internet gambling and they will investigate if called to.