My personal pet theory (for some of the graphs) is that someone is just loading up the book at many price levels with unlikely-to-be-filled limit orders to gain priority (for the rare occasions the market makes big swings). It's a perfectly legit tactic, that would help the market by providing liquidity and support against other players' big orders.
Some other graphs have perfectly innocuous explanations - for example someone probing the depth of the book or filling a big order piece-by-piece.
FWIW, you can't really pull a serious DoS against most exchanges. Most decent ones have pretty strict rolling-window rate limits. And no other firm is going to have much trouble processing the maximum number of orders one firm is allowed to place within their limit.
Even if you could get away with a DoS once (and I suspect you could), who's going to risk it? The exchange knows who you are and will punish you when you do.