I noticed this last week.
It’s hard to imagine it was accidental. Someone has gone to the effort of moving the stones to the side creating a clear route through. If a stonewaller had done that, the stones would have been laid out and graded, heavier stones nearer the gap, with receding rows of smaller stones, and finally the coping stones.
Now I’m not one to cast any aspersions but the gap is a convenient size for a quad bike.
It has been reported to the National Trust ranger who will no doubt be delighted to have to divert his tasks during this period of lockdown.