This is made precise in the state correlation diagram below. The ground state in the starting materials is the electronic state where πSS and πAS are both doubly populated – i.e. the state (SS)2(AS)2. As such, this state attempts to correlate with the electronic state in the product where both σSS and σ*AS are doubly populated – i.e. the state (SS)2(AS)2. However, this state is neither the ground state (SS)2(SA)2 of cyclobutane, nor the first excited state ES-1'=(SS)2(SA)(AS), where an electron is promoted from the HOMO to the LUMO. So the ground state of the reactants attempts to correlate with a second excited state ES-2'=(SS)2(AS)2.