Pavement fails for a number of reasons, but the main causes are original poor construction, oxidation by the sun and water erosion. First, when your asphalt was laid, the deteriorating portion could have had a poor sub-base or the asphalt was a little thinner in this area. Next, sunlight evaporates the oils and binders in the mix, drying out the pavements . Then hairline cracks start to develop allowing water to seep in and further erode the surface. (This is when seal coating and crack filling should be done). However, if left unchecked the cracks will worsen until water starts to erode the base material and eventually starts to cause potholes. Also, cracks will start to connect causing and "alligator" type cracking. This will allow, over time, pieces of asphalt to broke loose, exposing large sections of the deteriorated area. At this point overlay patching will NOT work if you are looking for a long-term solution. The area should be saw cut to full depth, the deteriorated asphalt removed and replaced with new hot asphalt material.