One thing we have seen (while I was working at Te Kete Ipurangi
http://www.tki.org.nz ) is that sometimes educational websites are hosted with discount hosting companies that cram on the same servers -- ahem -- "other types of educational websites".
If the (good) educational website is hosted there no more, but the
domain still points to the
server, there is a good chance your (good faith)
URL to a page on trigonometry will lead to akward places.
As TKI hosts a search engine full of educational links, we resorted to a periodic crawling of the linked pages, to check that (a) they didn't contain 'trigger' words from a list of words we imaginatively put togethere and (b) they had not changed too much (we had a fuzzy matching mechanism, and an old copy of the HTML of the page for that).
Having said that, I think this case is either a prank, a spambot trained to abuse Moodle (let's hope not -- wikis are suffering a lot from such spambots) or a trojan in his machine.