Like Steve said this could be many things. 1. If you used Durrock that was not rated for exterior use and you did not tape the seams, you will have a problem. 2. If you did not use a latex modifier in the thinset and grout, you will have a problem.
3. If you have less than 1/4" per 12" slope, with a drain, you will have a problem.
4. If you did not run "L" flashing around the deck between the Durrock and the tile, you will have a problem.
Hate to say it but you may have to tear out this out and start over... there is a product available from tile suppliers (not hardware stores) I do not recall the name at present, that is Orange plastic 42"wide which has a grid pattern to it, you thinset to the plywood and then thinset your tile onto. You can roll this up the edges and tape the seams. It is used in place of backer board on floors and is waterproof. (No I am not talking about 30 mil. pan liner). I just repaired a small entry porch that was above a basement using this product.