Why do airports remove/realign runways? When looking at Google Earth, I often find myself looking for airports. I often notice that no matter what size airport, over time many of them have abandoned old runways and built new runways in different directions. 

It doesn't matter between rural and urban, so residential doesn't seem to be the cause. Is it that the wind direction changed from decades ago? Did they decide that sunlight matters more or less at some point in time? Anything to do with Earth's magnetic field?
In some cases the prevailing winds change and certain runways fall out of use. Since airports tend to be land limited they may wish to use the land for hangars or other facilities so the runways are eventually decommissioned.
In other cases it could a noise abatement issue, many airports predate the property development that often surrounds them. It may be in the best interest of the local neighborhood to change the runway orientation to one that creates less noise and pollution over homes or businesses.
In some cases runways may be moved to comply with changing regulations as was the case at Aniak Airport, where an existing runway no longer complied with obstacle clearance regulations.
In some cases it also becomes expensive to maintain a runway that may not see much use and it will be shut down.

Magnetic shift generally wont cause a runway move but it may cause a runway re-number since the runways are numbered as per their magnetic heading.