The PCI-E riser boards are usually soldered by hand in China and while there are some with really good solering, there are others with crappy one and then there are the ones with the solder leaking and shorting multiple connections. Then these boards apparently do not go through any quality control and get shipped to customers... not the way to do things properly, but some of the miners ar also to blame as they are looking to save cost from everything and go to the cheapest option.