Here's a guess: dig into the bend or droop of one of those. I think you'll find the stem of an evergreen in there. Clear off enough snow, and the stem will straighten up... no more droop.
In other words, the snow fell so thickly and heavily (i.e., it was sticky, and didn't slide off the tree) that the tip of the tree bent over. Once that process begins, it tends to be self-reinforcing: a bent stem (closer to horizontal) offers more surface area for snow to fall on. (That effect diminishes once the stem bends all the way toward upside-down, but it's a one-way process: as long as the snow doesn't fall off, the only way the stem can bend is down.) The snow was sticky enough that it stayed in place even when it reached an upside-down position.
Details still to be worked out... why the bend seems to happen at a consistent spot along the height of the tree (though only in some trees); role of wind (direction of bend? Additional loading once in the upside-down position?); how much of the droop contains stem or was loaded when the tree was vertical, and how much is added by wind on the underside.