Concussion happens when the brain, for lack of a better way to describe it, ricochets around in the skull. A heavy enough body blow can do the job, as long as the impact causes that as well. It may have been the initial contact, or it may have been from hitting the ground.
Personally I did not see anything wrong in the tackle itself,and whether it was a Bronco or Storm or Rabbitohs or whatever else player I would be saying the same thing. However, the match review committee has drawn a line in the sand on the issue, and I would expect Te'o to be getting about six weeks- not because there was necessarily anything wrong with what he did, but because the standard has been set and his appears worse due to Groats' concussion.