Interesting! I've had kind of the opposite experience. Sometimes, but not always, grass fed beef develops a light gaminess but it isn't heavy or objectionable. Additionally, it goes away with just a little wet or dry aging -- it is more prominent in fresh butchered meat. On the opposite side, finished beef (grain fed for the final period of its life) has a "sweeter" initial taste and a more mellow flavor following any aging. This is what we used to do on the ranch for critters we kept for our own consumption.
Any chance you got the two reversed? If not, I'd use it as an excuse to do the whole thing over again
Not everything in life has an explanation.
Hub