So there is an attraction then. I wonder (clutching at straws now) if homing pigeons use something like a ley line or 'natural' points of guidance to find their way home? Any pigeon fanciers amongst us?

Underground water source surely creating a moist soil...as midges need dampness for egglaying....this damp spot would also give a more verdant greenery which might explain the ponies preferring the grass there.
Not a proven suggestion...just an idea.