I've had this happen to me on a couple of occasions, I think it's when there is a run on a coin and the server can't handle the traffic; it's usually not too bad but can be a bit worrying - a victim of there own success.

On another topic, just noticed that they changed, on the 'Deposits & Withdrawals' page, the "Hide zero balance" (IIRC) to "Hide small assets", this I don't like as it means I have to view all assets to see the forked BCX and SBTC that I have - please Binance change it back.