So sad to hear about the two hogs, it must have been upsetting for you finding them like that. It isn’t really possible to say why they died, but hibernation is probably the most dangerous, natural, time for them and not all will survive that. Hogs don’t necessarily hide when they are ill – they will often try to get into the sun to warm up. Seeing hogs aparently sunbathing, even in the Summer, is an indication that they aren’t well.
It sounds as if it might be too late for those two, but if you ever found any other dead hogs (which hopefully you won’t, but others may do and could potentially read this) you could consider reporting them to Garden Wildlife Health https://www.hedgehogstreet.org/garden-wildlife-health-gwh/
For the future, re. the neighbours dog – sadly if a dog has had a hog in its mouth it is very likely the skin will be punctured. Such punctures are difficult to see, other than by experts, but if one is there the hog is likely to get an infection and could die as a result. So if a hog has been seen in a dogs mouth, it’s best to get it checked over by a hog carer/rehabilitator or a vet in case it needs antibiotics.
I mentioned the possibility of avian flu to some hoggy people, but they hadn’t heard anything about hogs contracting avian flu. Hopefully that continues to be the case.
There are some reports of hogs out and about a bit early this year, although there have also been quite a few non-hibernators – not always easy to know the difference at this stage. But males, in particular, tend to emerge early March which is creeping closer alarmingly quickly! Fingers crossed that most of the hibernators return safe and well and have a successful Hog Season this year.