There are very few instances where a window frame has become damaged by rot any significant distance above the cill. Such rare instances have four main causes: when water gets into the drip bar or the transom; when a closed opener sits tight on the transom leading to capillary action; when the opener loses the throating, usually the result of swelling, so someone shaves off the bottom rail so that the opener fits again and doesn’t put in a new throat; or finally when the throat has become filled with layers of paint over the years. There is a reason why the throating should be present all around the opener: it is to stop capillary action taking place.
The most common area where you will see rot is in the corners of the cill and in the first 6” on the frame jamb. It can be caused by the above mentioned faults, or by the pooling of water in the corners coupled with a poor paint surface. If the paint cracks at this crucial joint and water seeps in, the water has no where to go and no means of drying out, perfect conditions to allow wet rot to take hold.
Other causes of wet rot can be: poor quality paints allowing moisture penetration; painting timber with a high moisture content; timber of inferior quality (usually not applicable to pre 1970s frames); shrubs growing too close or against the timber; and gutters that leak.