Particleboard (you sure it isn't OSB?) is a real no-no under tile. If it gets wet, it swells up like a sponge. That movement will cause the tile to break loose. OSB, although not the first choice, may survive. But, as you've discovered, not putting thinset UNDER the cbu is also not good. The idea of the thinset is not to hold the cbu down, but to fill in any (hopefully) minor imperfections (the screws hold it in place), and ensure that the tile are 100% supported. Did you use the special mesh tape on the cbu seams? BTW, while more ply is never a deterrent underneath a tile install, over planks, the minimum is 1/2". For your next project involving tile, check out www.johnbridge.com.