What causes the forest fire is not the presence of water, but rather the absence of it. Water puts out fires and prevents wet items from burning - you cannot hope to burn a wet towel unless the fire is there for long enough that the heat causes all the water in a particular area to evaporate.
However when the object is dehydrated and then overheated, or if a spark lands on it, it will burn easily. A drought resulting in many dehydrated branches and leaves would increase the likelihood of a forest fire.