Yeah, I did look closely at the floor prior to putting the 3 x 5 sheets of cement b-board in place. But they are all down completely. The tiles that arew teetering are not mortared in yet. The first tile that teeters badly is next to installed tile (no grout yet), and about 18" of backerboard on the other side to the wall.
The second tile that rocks is in an area where no tile has been set. Only backerboard. Must be some kind of "hump" in the b-board but I cannot really see it. I tried using a tile on edge, upright, to see specific spots where it pivoted, then used the 5" grinder to work that spot down. Not much luck, but could tell a slight difference after I grinded.