Exploiting the temporary absence of the black queen, White creates a mating net. In view of 6 Qf6+, Black resigned.
1...a6 seems preferable, avoiding a weakness on a7, although after 2.a4 Rd6 3.b5 a5 4.Bg4 Rf7 5.Rc1! Bd7 6.Qa3! Qe7 7.Rc7 White keeps strong pressure.