This is a BVH file issue then. Not all BVH files are created equal. Some vendors that provide BVH files for free on the Web have files with "issues".

The next step will be to try a different BVh file, preferably one from another vendor. If you have Poser, you can also animate a figure and then export the motion as BVH an import that into Carrara. Carrara and Poser play fairly nicely together regarding motion files.