Why do tiles crack?

Answer 1

One reason grout between tiles cracks is that grout over time will dry out

There are several reasons grout can crack and almost always has to do with movement.

1. Gaps are too big - joints that are over 1/8" thick should not use standard grout its simply not designed for joints that big, use sanded grout for anything above 1/8"

2.Weak or flexible surface - If the surface your walking on has any flexibility or weakness it it will cause cracks in the grout as the surface underneath flexes or moves. e.g. Plywood

These two are the most common reasons for cracking, severe temperature changes improper filling of joints or thin mortar can also cause cracking but are more uncommon.

If you hear crunching sounds under the tile the mortar is most likely cracking and your tiles will be soon to follow.

Answer 2

If a force is strong enough, it will crack the tile. Fast movement of the earth might cause tile to crack also.