Why resign?

Why? White Kg1,Qc2,Rksd1,f6 Ba2 Nd6 p's b5,e3,f2,g2,h3

Black Kg8, Qb6, Rks d8,f8 Be8, p's a5,b7,f7, g7, h7... White rook on f4 just took Knight on f6 and Black resigned...