Based on our experience,  DDoS attacks mostly happen due to following reasons:

1) Your business rivals or enemies of your hosting want your business down.
2) There is malware/trojan/script in your server.
3) High bandwidth usage on the server side due to some activities done on purpose or unintentionally (e.g by trojans).

You can refer to the articles below that cover more reasons why DDoS attacks can happen:

[ http://www.slideshare.net/intruguard/10-most-common-reasons-you-get-d-do-s ]
[ http://en.wikipedia.org/wiki/Denial-of-service_attack ]
[ http://www.prolexic.com/knowledge-center-white-paper-four-reasons-why-ddos-attacks-happen.html ]

Do get back to us if you need any assistance concerning this topic.
Thank you.