The number 1 reason is trash from leaves, pine straw and shingle granules. Once they stay start to accumulate in your gutter, they impede the flow of water to the downspout. Over enough time unchecked, rain water will start pouring over the gutter.
#2 The second reason for gutter problems is small. (2"x3") builder's grade downspouts on the gutter system.system. This is especially true when your home has upper gutters dumping onto lower roof gutters.
#4 The last reason for an overflowing gutter is inadequate amount of downspouts for the gutter system. As today's homes incorporate more intricate designs, managing rain water becomes involved and complicated. It is imperative to ensure proper drainage from the roof to the ground by maintaining an adequate number of 3"x4" downspouts for the gutter system.
Once these (4) items have been inspected, evaluated, repaired and modified, you can have the piece of mind knowing your gutter system is providing you the best possible protection.