is hard to tell who is chasing who or why 
(ROOT (VP (VBZ is) (ADJP (JJ hard)) (S (VP (TO to) (VP (VB tell) (SBAR (SBAR (WHNP (WP who)) (S (VP (VBZ is) (VP (VBG chasing) (NP (WP who)))))) (CC or) (WHADVP (WRB why))))))))
(ROOT (VP (VBZ ) (ADJP (JJ )) (S (VP (TO ) (VP (VB ) (SBAR (SBAR (WHNP (WP )) (S (VP (VBZ ) (VP (VBG ) (NP (WP )))))) (CC ) (WHADVP (WRB ))))))))