Usually this is due to a poor sub-floor like those found in many older homes. Some older flooring reacts to the humidity changes by expanding and contracting, sometimes causing cupping or crowning effect on the strips. When you walk on the floor the pieces might flatten out and cause a rubbing effect that gives a creaking sound. In newer floors it can sometimes be a result of shifting of your floor because of changing humidity. This can be avoided by maintaining an equal humidity level throughout the year.
Hardwood floors, all of them, will eventually develop some sort of squeak in them. The reason behind this is that they are nailed to a wooden structure called a sub floor made of wood underlayment and joists.
When the wood expands and contracts with the various seasonal humidity changes, the holes around the nails also expand and contract. As we use and walk on the floors we continue to loosen them by jiggling them ever so slightly each time we use them. Your floors may squeak more in one season than another. For most homes, they squeak more during the heating season, which tends to dry out and shrink the hardwood strips.