Check the joist structure when you get home, too. Joist type, size, spacing, and longest unsupported span.
Is the grout cracking random, or are the cracks mostly in one direction?
Any cracking in any tiles? Any tiles appear loose or sound hollow when you bounce a golf ball or hard screw driver handle or similar on'em?
Did your installer fill and tape the joints in the CBU before tiling?
Have you been a truly good person since the tiles were installed?