Have not read the full text of the article, though the following is part of the synopsis:

"Sudden, uncontrolled movements called dyskinesias—a common side effect of treatment for Parkinson’s disease— are a result of excess serotonin cells in transplanted tissue that trick the brain into releasing dopamine, suggests a new study of two Parkinson's disease patients that received fetal tissue transplants over a decade ago, researchers report in the June 30 issue of Science Translational Medicine"

I guess the question is how do excess serotonin cells "trick" the brain into releasing more dopamine?

from another source:

"...In the new study, using brain imaging, Politis and his colleagues found that the dopamine neurons that decay in Parkinson's disease were restored and functioning in the two patients. But they also found abnormal levels of serotonin neurons within the transplanted tissue.

The serotonin neurons were releasing dopamine, which, when coming from the wrong source, caused the jerking movements.

When patients were given a serotonin receptor agonist, a drug that prevented the serotonin neurons from firing, the involuntary movements ceased..."

http://www.nwpf.org/News.aspx?Item=3397