OP is totally correct. It *does* have to do with snaps that you have enabled. If you don't have grid snapping enabled, you can't reposition the dimension to anywhere but the other types of snaps defined. In trying to drag a dimension outwards, away from all other intersections/ends/objects/whatever, not having grid snapping enabled means that there is nothing to drag this toward, since only the grid exists outside the objects. This is a fundamental concept, which it turns out applies to dimension objects, just like all others.
It's actually a bit annoying, I have to turn off grid snapping to get my dimension snapped correctly, and then turn it back on while dragging it outwards away from the drawing/objects.