It's not so much an 'attack' as it is a rapid series of accesses to various links on the site. Yes, they probably check your site too, but if you have pages that don't create more links for historical pages, you have likely no issues.
Each wxforum.net page has links to other pages on the forum and if too many requests are made at the same time, the number of PHP threads available is exhausted and you receive the 500 error as Apache is unable to launch another thread to satisfy the request.
The Google, Bing, Yahoo bots are well-behaved (no more than 4 bots crawling at the same time and frequency limited) -- the rogue bots can spew a stream of requests which back up the PHP instances on the webserver, and they tend to ignore the conventions of using robots.txt and in-page <meta name="Robots" content="index,nofollow"/> on pages. They get blocked (by CIDR) in .htaccess.