The server is huge, it's one of the biggest Minecraft servers around, and is the biggest economy, with this big of the population, you're going to get people saying thing such as "Give me op or i'll DDOS" Of course they can't accept that; hence they do DDOS the server.
In computing, a denial-of-service attack (DoS attack) or distributed denial-of-service attack (DDoS attack) is an attempt to make a machine or network resource unavailable to its intended users. Although the means to carry out, motives for, and targets of a DoS attack may vary, it generally consists of the efforts of one or more people to temporarily or indefinitely interrupt or suspend services of a host connected to the Internet. <----from wiki.
