if hypothetically a horse has suddenly started box walking, nothing has changed in its diet or routine, could it be down to an under lying health issue? if yes what sort of health issue is it likely to be?
my old gal box walked on our yard because we were between two major motorways and it worried her, especially as the headlights shone right in to her stable, we moved stables and she was still funny but if I left her door open she would spend most of the night in there. silly little things, youdon't notice can make huge differences.
You say nothing has changed at all for your horse but this is from YOUR point of view only. Something important HAS changed from your horse's point of view! Now you have to work out what. New neighbour, old friend gone, sheep in a field 5 miles away that weren't there before, trees taken down, hedges cut, neighbour being fed/boxed different times, lost weight, gained weight, work changed, more competitions than before etc etc etc. A million things. It's down to you now to look at everything from your horse's point of view. Break it down into tiny pieces and analyse it as if you were a horse. It could be a pain issue or box walking could be something that your horse used to do before you had her and has now re-started due to stress. Stereotypical behaviours never totally go away so I bet she's done this before. Whatever has caused it may not be a sudden change but may be a build up of things she's been struggling with for some time but that you've missed. Now she's shouting, you have to hear her.