I have had that problem in the past and had it go away when I restart the CK. I have never been able to figure out why. It might be one of those odd key sequences they use.
I've restarted several times, and it persists. :/
Maybe something in the SkyrimEditorPrefs.ini? Back it up somewhere then delete it. Let the CK make a new one, then see.
Just tried that and it still does it... Though, it does appear that it's creating them a certain distance off the ground now, not exactly right at my camera, it was just zoomed in closer. If my camera is 50 ft in the air, and I try to place a rock, it's about 15ft above ground along the axis from my camera to my cursor's location.
Also, it appears to mostly be happening in one exterior area... Can anyone think of something that could have happened there? I went to several other interiors and it didn't occur. But in these few exterior cells it does it for no reason... There is nothing blocking them, no collisions (with F4), no fog or FX or water. Nothing... I've finished most of the area (It took forever...), so as long as it doesn't happen anywhere else I guess it isn't really a problem anymore. But it sure is weird...