The Vertical Rail

Our farm is on a hill and a partial section of the pasture is at the bottom of that hill.  Along the perimeter of that section, the pasture is enclosed by an old fence line of railroad ties.  It's more like a wall than a fence and has been decomposing at a rapid pace.  Near the top of the hill where the wall/fence runs, many of the ties are collapsing.  However at the bottom of the hill along the road, it is holding up in fair condition.  Fair as opposed to the rest of the fence.

Today I looked out my window and noticed one of the ties along the top of the wall was lying in a vertical position.

What made this odd was two things. The first being that the obvious culprit would be the horses.  They will often reach over the wall/fence and eat the grass on the other side.  However when they do, this is what they do to the fence as seen further down the wall.

The tie simply gets knocked over.  Makes sense, the horse puts it's weight on the tie rail and over it goes.  But this tie was moved in a vertical position to the wall.

It is in an awkward position for a horse to have maneuvered.  But what made this even more odd and brings me to my second point is that the horse's haven't even been in this section of pasture for well over a week.

So if it wasn't the horses, who or what could have done this?  I've pulled all the rails back in place and will watch and see if they move again.