If we exclude global warming as the usual reason for everything these days, I would bet my shirt on dogs being the answer, either with or without their owners’ connivance.
Deer, like most animals, are creatures of habit. They always jump fences in the same place, usually where it’s a bit lower and there is a good take-off and landing spot.
They then do it automatically, as part of their routine.
When alerted or alarmed they will still take the regular, known route out of the field but if really terrified, the routine breaks down and they just run without thinking.
Coming to a new bit of fence, usually in a corner of the field where they can no longer out-run the dogs, they sometimes make an error of judgement and get their back legs caught between the two top strands of the fence.
At that point they may be ripped to pieces by the dogs, despatched with a knife by the dogs’ poacher owners, or the dogs may take off after another deer and just leave that one hanging in the wire.
It is all most unpleasant and not easy to deal with unless you have the time to try to catch the perpetrators in the act.
Hunting with dogs is illegal and I am sure the police will be interested to receive evidence of an offence, especially if you can give them car registrations.
These are often false, but it is worth phoning the police, who can check instantly whether the thing is road legal and respond accordingly.
Remind them not to come with sirens and flashing lights, which only warns the offenders, and gives them time to escape.