https://gyazo.com/158036775ebfbbb5f4b7c11db39d4c8e
This is how it imports walk animation
Never seen that before.
Something to try select all of the bones and using rotation set all of the values to 0 0 0. This should snap the rig back into its default pose. Select the root bone and import the BVH into the hierarchy. If it does the same thing then assume that the file is broken and find and try a different walk cycle. (tons out there)