those are Market Maker games.. often large orders will show up on the bid or ask to influence the retail inverstors to move the price.. if there is a large bid then some traders will buy the ask thinking there is alot of support.. but then the bids disapear and there seems to be more sellers now on the ask, so traders will begin selling, and the big guys flashing these large orders just made the spread... they design trading bots to try to move around the weaker hands and make a bouncy market.. its normal stuff.. its actually a good sign, that ETH is healthy and worth the big guys writing these trading algos for..