@Reyn,
Most (as in more than 99%) hacks are automated and opportunistic (for money, it used to be for curiosity or challenge but these days it's all about making money). It was running popular vBulletin software with a vulnerability and then got hit by an automated exploit.
The motivation of the exploit was to make money, through malware etc, but the targeting of the particular forum is just automated, like a virus. It runs a popular (and frequently hacked) forum platform and a malicious script found it and infected it with code that seeks to infect its users.