My first question would be are you following a public footpath ? the way you refer to open fields sounds like you're get going where you like.

If it is a public footpath then it could be that the shooter is very close has just shot the rabbits but is wary of collecting them (carrying a gun) while the public (you) are in the area, he doesn't know how you will react.
Equally it could be poaching so the shooter doesn't want to be seen, or pure vermin eradication where they're left for predators.
The wound sounds too much for an airgun.

I suppose it could be RHD/VHD where the shooter has seen that they were not acting normally so has dispatched them but decided they are not safe to take, the dogs might have been able to detect/smell the illness.

A couple of months back I found 2 dead rabbits still soft, unmarked, I skinned one looking for a wound thinking it was my idiot neighbour poaching but there was no wound & no sign of any internal haemorrhage.