There are people with botnets that think mining bitcoins on them is a good way to make money. The pool owners disagree with this and have been cutting them off, partly because the number of low-hashrate users is disruptive to pool operation. You figure out the rest.
hmm ok, i get it. So what is exactly being ddos'd. Is it on ip level or application level? or both? Too much ip traffic to handle or is the server bogged down by requests?