The threat is Ne4+, Kh4, Bg3#. Rb4 instead of Rb7+ doesn't help because of Nxe6+, Kh4, Kh6 with (almost) inevitable g5#. It's very nice variation in fact (continuation after Kh6): Rg4, g5+, Rxg5, Nxg5 and white is in zugzwang as only g-pawn can move but it leaves the field f3 uncontrolled, so Nf3# next move