What causes a dyna wide glide to wobble at 70 plus mph?

Most Harleys do. The reasons AFAIK are due to the soft suspension, engine wobblynes and frame geometry. There are some products, such as the True Trak that aliviate this problem. I put a fairly cheap steering damper (120 USD ish) and it works quite well. But first one has to check if there is something wrong with the bike, misalignments etc. Heavier fork oil and harder progressive fork springs also work wonders for cornering stability, and it´s a cheap mod.