To be fair, tor is a really crappy network to orchestrate a DDOS attack. On the other hand some sites just can't take much traffic, especially if it is a consistent inflow of requests. It may be that somebody just used tor as a proxy to crawl your website and it crumbled.

I happen to speculate this because I may have used tor for web crawling, which may have caused a specific site to crash before I cut back the crawl rate. Punishing tor for such incident is mostly FUD.