It certainly isn't the only reason why it happens, but Jan's answer is probably the most common cause. Things should not be turned on in the wrong order. The simple causes are usually the ones that 'get' you the most often and usually the ones you tend to 'forget' (at least me, anyway). From there, it starts getting a bit more complicated. Simplest way of finding the problem is by substitution. ~IF~ you happen to have the proper thingy to substitute for each device in the circuit. But what if you don't have the thingy you need to substitue? You do what the rest of us do, ask questions and make guesses - LOL.
Good luck.
- 'Doc