IF the quoit had fallen in the past and been re-erected, then I suppose anyone (such as people like ourselves) interested in the ruin could have done it and at any time. And if it was lying around for a considerable time it is possible that certain stones may well have been reassembled incorrectly. I'm currently working on that puzzle because as I've said before that is the feeling I get when I look at it through the eyes of a person who has been in the construction industry all his working life.
There had to be much more to the 'mound' originally though, that, I believe is quite obvious. If the uprights are barely in the ground (just enough to station them) then the soil/stones banked up against them would have to be quite substantial to hold it all in place to be able to (slide?) the capstone into place. This suggests to me that the orthostats would have been certainly completely covered inside and out to even give this a chance of succeeding. These were ordinary folk of their time who would have recognised the easiest way of achieving this and completely enveloped them in soil/stone to stabilise the whole thing first. Once the capstone was on then the side slabs could have been put into position (if not already there) as the soil/stones were gradually removed. No doubt a ramp would have been part of the original build and at the same inclination as the capstone.
All of that apart it is the most wonderful thing to just stop and stare at and I feel priviledged to be able to do so whenever I choose after a few minutes drive.