Add to trim, the higher seat position due to a new cushion into the mix.
His perception of the correct take-off attitude could have been thrown (thinking it was normal when it was in fact tail low) and add to that the stick feel with rearward trim (he would have felt he was pushing forward with some resistance so may have felt the tail low attitude was the normal TO attitude).
It looked right, it felt right...
All supposition of course but I know that adding cushions or trying a new seat position sometimes results in pilots stuffing landings when they misjudge the 3 point attitude in a tail dragger. I could see how it could trigger a tail low take-off as well.