Can't explain, but can tell you you're not alone - I often move fences, place paths and then put the fences back.
But from the picture I wonder if you have uneven land too right next to the path, so that it can't build flat - is it still restricted after you press 'U'?
And further to that; if your path goes even slightly uphill or downhill, then you will not be able to build off it at right angles for that reason too
It is a little arbitrary and a little frustrating; but you normally find a way eventually

(don't get me started on why it's impossible to lay a foundation so you can put bins on slopes

)