It's happening because someone forget to change the A record at their registrar. It happens more often with shared IPs and people who have unused, old domains that were previously functional. It really isn't a bad thing for you. I wouldn't worry about it.
Or they are doing it on purpose. In 2004 or 2006 elections, some guy had registered janetreno.com (running for Governor of FL) and just redirected it to her real election site. People linked to janetreno.com; thinking it was her site; when the election got up and running...he switched the domain to be a bash janet reno site instead. And since people linked to it, it got lots of traffic. Great idea. But I'm not so sure I'd want someone capitalizing on my domain name and trying to garner free links or siphon off traffic, even if its unlikely to have a long term impact.
This really raises an interesting point, or question rather Lorien. I really wonder what the legal ramifcatiions of this are. I mean is it really illegal to point a domain to another domain? Is it really any different than simply linking? I know there are currently thousands of attorneys on DP. Hopefully one that actually went to school to be a lawyer could answer this question. I know of no precident or case law that really defines the legality of this particular issue.