PCB burning can be caused by different things, but usually a short circuit or capacitor failure. The latter is extremely common, especially in power supplies, typically caused by bad capacitors ( a global quality problem right now-- I have replaced bad capacitors in my TV and refrigerator and there are numerous reports of how big/bad this is) and/or bad design decisions (erring to low end in capacitor value, rating, etc). Both of these problems are driven by the mad race to get things cheaper-- although African "politics" also plays a huge role in bad capacitors.