Yes I am a glider pilot - paragliders. You may be quite correct about the dunes, and thank you for the information that they can cause wave effects so far downwind.
I was not alluding to thermal effects in the dark, although they can happen. The hot air is trapped in the trees during the day, especially if there is light breeze. If the wind strengthens this can disturb the stagnant hot air and cause it to burst from the trees; restitution thermal bubbles of a sort. The orographic effect can happen at any time of day, and if you can not see the forrest the sink can be surprising at low level as you pass over the boundary of the tree. This can also happen in daylight and any orographic effect combined with any thermal effect can make the last 500' quite interesting.
I have experienced exactly the same effect at other airfields with large woods in the undershoot: no sand dunes or other sea breeze effects. The same is more obvious at PMI 24L and VLC 12. In those places there are large areas of dry sandy hills and that can also make going down & slowing down challenging if you leave the drag configuration too late. And that is even more surprising in the dark, just after sunset, as the stored heat is released with a gentle breeze over the rising terrain.