There's no way you or anyone can really have any idea as to what exactly gave Horton the concussion.
The hit was to the area of the collarbone. It was the impact on the ice that gave Horton the concussion.
Bert got one by turning his head and hitting his chin as he ran into Heatley.
When you get hit that hard and your brain gets rattled that much, the hit alone and head snap certainly could've caused it as much as the impact to the ice.