Daeseong, The writer could have written the sentence using a relative clause: -All hibernators periodically rouse themselves for hours to days, which is not understood behavior since it uses up precious energy stores. Instead he chose to use a noun phrase in apposition to an independent clause. Let me explain about apposition. In apposition the second element serves to modify the first element. Commas are used, but no conjunction is needed. A noun can be in apposition to a noun. -Bob, my best friend, eats pizza every day. A noun phrase can be in apposition to a noun. -Bob, the man who is my best friend, eats pizza every day. -A noun phrase can be in apposition to a clause. -Bob eats pizza every day, a behavior that is not hard to understand since his Dad owns a pizza parlor. -All hibernators periodically rouse themselves for hours to days, a behavior which is not understood since it uses up precious energy. -The growth of the industry is far more broadly based than before, a fact which has enabled it to survive the financial crisis.

This is not simple to answer, but it is because the sentence uses "a behavior" instead of "this behavior". Because "behavior" is followed by "which", it makes it clear that "a behavior" refers to the first clause of the sentence. If you were to write this differently, you'd end up with "All hibernators periodically rouse themselves for hours to days. This behavior is not understood," etc. The same structure can be used for something like "I just read Harry Potter, a book which I didn't find very interesting."