DDoS is usually facilitated by viruses on many many computers. The offending nonsense requests can be going on from your computer, and if you weren’t looking for it, you wouldn’t be aware of it.
Not to mention home routers, internet-enabled DVD players, refrigerators, webcams and all the other crap that's being hooked to the internet. This other stuff, the Internet of Insecure Things, is potentially worse than people's computers because the insecure software in these things never gets updated when vulnerabilities are discovered.