Cracks around window edges are worrying. They are an indicator that you need to take action. There are many possible causes, several of which are outside your control. If your windows are still under guarantee, you may have some legal redress. If not, you need to seek professional advice about remedial action.
Buildings are not static. They move as the ground shifts. Fortunately, these movements are very small, in normal circumstances. According to members of The British Geological Survey, measurements of ground movements can be achieved with radar interferometry. Ground movement is especially noticeable in former mining areas, due to subsidence. These movements can cause cracks in buildings and around the edges of windows.
If you have had double glazed units fitted and cracks are starting to appear around the edges, this may be because of faulty units or incorrect installation. Some double glazing companies offer a lifetime guarantee. Others offer ten years or less. Contact the double glazing company responsible and explain the problem. If they fail to respond and they are members of the Fenestration Self-Assessment Scheme (FENSA), you need to get in touch with them. According to FENSA, 92% of windows in the UK and doors are covered by their certification scheme.
Earthquakes are not as rare in the UK as you might think. Britain is squeezed to some extent by the Earth's tectonic plates. Melting ice sheets also cause deformation. All of this activity causes occasional earthquakes. Even though they are not nearly as severe as in some parts of the world, they can cause cracks around window edges. The effects of the 2001 Melton Mowbray earthquake included widening cracks to window frames.
Wood is an organic material. If you have wooden windows, the wood will absorb and exude small amounts of water, according to prevailing weather conditions. This causes the wood to expand and contract slightly. The expansion and contraction of wood can cause cracks around the edges of windows. Whether your windows are wood or uPVC, regular loud sound and other mechanical shocks can also cause cracks around window edges.