Looks like a spam outbreak

rAiN

a simple exploit injection in database can cause this, admin really needs to save and backup database weekly and revert back to before the spam break outs.

this can be easily done by changing a file name to inject.xss to inject.xss.jpg and upload them into the forum image uploader.

since we dont know what the "cracker/hacker" used the file to contaminate the database but that is how they usually get into database. or at least one of the way.