Question

why does the forest catches fire

Answer 1

due to lightning..........

Answer 2

There are a few ways wildland fires start, but can be generally broken down to either by nature or by man.  A lot of the "hard to access" fires are started by lightning strikes.  Generally these occur during dry storms, but not always.  Others start by either the carelessness of man (not assuring campfires are out, burning during dry conditions) or by arson (intentionally set).  A few are set by forest rangers as "controlled burns" that get out of hand.