That I've seen it's kind of unknown, however this study would suggest that it might be inhibition of colonic motor signals. It seems (decently?) established that it is a colonic motility issue and not upper gi tract. http://www.ncbi.nlm.nih.gov/m/pubmed/9514436/
This may not be the actual mechanism, but think of both the vasodilatory effects and the constipation as the result of blocking calcium channels which are required for Muscle contraction. Less calcium = Lower BP and lower colonic tone, which leads to constipation.
Unfortunately, I cannot explain why and how it caused constipation but I can suggest how to relieved it. You can drink more water to add fluids in your body. If it is not effective then try an herbal supplement that is best for it. Like me I have this Digestic by Mimonis which is very good in treating constipation. It has done a big change in my life so I am taking time here to help others also.