As mentioned before most likely a lightening strike. See
https://www.theguardian.com/world/video/2016/aug/29/300-wild-reindeer-killed-in-lightning-strike-in-norway-videoA much bigger mortality event.
Lightening does not necessarily cause ground damage, nor does it have to have struck directly between the deer, it could have struck to one side. Yes, I've seen significant ground damage as a result of lightening, but where the strike has hit resistive material, e.g granite with blocks blown out, or saturated frozen ground where it was like a giant pig had been rooting with its tusk as the current tried to find its path to earth. But if lightening always caused damage the top of every hill would be a right mess.
Wouldn't bother informing the estate, they will just say thank you very much and then count the mortality against thier cull targets, and we are trying to get the blinking deer numbers down!
Anyway did you cut yourself a steak or two?