Don't push the palm keys in when you play. Bad technique. Instead, pivot your hand around where the thumb is resting and PULL DOWN the palm keys. The key will catch against your palm and open up just fine. The motion of your hand is like a mini karate chop, and the D key just happens to be in the way and get pulled down by your palm. Once D is open, just use the first and second fingers to open the other palm keys. The horn holds still because there is no lateral pressure against it that isn't countered by its anchor points (the neckstrap and thumbs). In the case of a D, there's no lateral pressure at all, really.