Sounds to me like 'submissive urination'. I found a website that helps explain it...it's easier to give you this than to try to explain it myself!
http://www.usask.ca/wcvm/herdmed/app.../suburine.html
There are lots of other websites out there that give you tips on helping to deal with it as well. Also, I would have her checked out at the vet to rule out urinary tract infection.
My last pup suffered with submissive urination for a few months after we adopted her from the SPCA. Once we learned WHY she was urinating, it didn't take long to help her eliminate the problem. She would have the rare accident in years to follow, but was pretty much free from it for the rest of her life.