As you might suspect, the owner plugged the car in and then walked away, according a police officer who spoke to NRK (in Norwegian but translated with Google). Local authorities are investigating along with Tesla. The local fire department was able to douse the fire using foam rather than water, which would have made the fire worse.
A few other Tesla electric vehicles have caught fire (see here and here), but far more gas-powered cars burn up every year. Authorities around the world have prepared for the shift to plug-in vehicles and the potential risk to first responders with specialized safety seminars for years. Other plug-in vehicles have caught fire when wired to home chargers, but we cannot remember any previous incident where a Tesla vehicle caught fire while connected to a company Supercharger. Wikipedia lists one Tesla fire while plugged in at home, but otherwise has no other Supercharger fires.
As for the potential fallout of this fire, the investor website The Motley Fool says that, "While it wouldn't hurt to keep an eye on the investigation, there's no reason to worry at this point." That seems right.