One of the prime motivations for DDoS attacks today is hacktivism. That is, some individual or group is trying to inflict revenge or punishment on the targeted company because of political or social beliefs.
One of the more famous uses of DDoS as a hacktivist tool is the take-down of Sony Pictures' systems in retribution for the planned release of the film "The Interview." Presumably a group loyal to the Supreme Leader of North Korea didn't like the way the movie depicted their head of state and they let Sony know by taking out the film company's websites for a few days.
In March of this year, the censorship watchdog group GreatFire.org pissed off the Chinese government. GreatFire provides alternative ways for Chinese individuals and companies to get to websites that are censored in their own country. This didn't sit well with the Chinese government, which is suspected of sending a massive DDoS attack at both GreatFire and GitHub, which hosts some of GreatFire's mirror sites. (Read more about this here.)
You don't have to anger nationalist governments in order to face retaliation by DDoS.
The feminist blog Femsplain was attacked on March 8 – International Women's Day. Although no one claimed credit for launching the Low Orbit Ion Cannon (LOIC) attack on the website, a science blogger blamed it on misogynists who didn't want so much attention focused on women. The irony is that Femsplain used the attack to get media attention, thereby getting more people to learn about the blog.
In March, the Madison, Wisconsin, police department felt the wrath of the hacktivist group Anonymous with a DDoS attack in retaliation for the shooting of biracial teenager Tony Robinson Jr. by a white police officer. The Madison PD website, along with its email servers and in-car laptop servers, were DDoSed by Anonymous and were intermittently going offline, according to the police authorities. Anonymous posted a video on YouTube to share a message demanding that the Madison Police Department and its chief, Michael Koval to, “release all audio and visual recordings from the scene immediately, and arrest the officers responsible for murdering Mr. Robinson.”
The state of Indiana website was knocked offline by a group taking up another person’s protest against the Religious Freedom Restoration Act that was signed into law and eventually modified because of public protests. The group @YourVikingdom targeted the state's website after someone suggested that a campaign against the state be mounted in response to the new law that was seen by some as being discriminatory. The website was offline for just under an hour and continued to run slowly for several hours. @YourVikingdom has a reputation for attacking websites for the fun of it. I'm sure the state employees who fought to keep the web servers up weren't laughing.
With DDoS tools being readily available to anyone with Internet access, we're sure to see these types of attacks used more frequently as punishment or protests. Hacktivists justify the means by believing the results are righteous, when in fact they are merely the manifestation of the crime of tampering with someone else's computing resources.