There are more and more integrated parts on motherboards like sound card, Network Card/jacks, USB, etc. Then there are all the people who try to get a new motherboard and run IDE drives, even though the new SATA interface has been out for 3-5 years by now. Also you will see people try to run Memory that is incompatible for the motherboard. Then there are the motherboards that people either overclocked or they tried to power it with the wrong Power supply. Along the way people also try to screw down the motherboard directly to the metal plate or there is a connection between metal parts that ground out.
Some motherboards are just bad when they are made or they use lousy components or they use the wrong components altogether. Capaciters have different ratings and maybe they get mixed up or the supplier sends the incorrect or faulty part and it gets used anyway. In some samsung TV's it was like a lot of the wrong or capaciters that did not meet the specifications were used. Often it was in the Capaciters in the power circuits. They work for a while, and then they go bad.
It seems like the inferior motherboards are often the cheaper ones. Then once in a while some model is just a lemon. There are an awful lot of things that can go wrong with a motherboard, the least of all is just plain old operator error.