I would imagine that the investigation will focus on what ATC (ground) told Westjet to (and did Westjet follow their instructions) and what the parking policies on the ramp were supposed to be and did the RCAF properly park the Hercs. However, in general, it is like a car accident in a parking lot: the person in control of the moving vehicle is ultimately responsible to ensure they have clearance all around and don’t hit stationary things.
After that, the lawyers and Westjet’s insurance company gets involved to figure out who pays whom (The Crown is self-insured) and how much.