Hi, I forget what the exact cause was, but awhile back, I noticed that for The Guardian in Bing, the description blurb started out with "The Guardian, a far left wingnut rag..." - I told them about it and they had this recurring problem fixed. It was some sort of cache poisoning somewhere that is out of your control. So if you are sure you have not been hacked...

Google Webmaster Tools might have some help pages - do you have an account for managing your domain? IIRC, there might be a field somewhere where you can set this description. I know you can manually set this in some fashion... You have to prove you own the domain etc first. And check or get a Bing Webmaster type account as well.