The frontier-orbital method explains how this reaction works. The sigma bond in the reactant will open in a way that the resulting p-orbitals will have the same symmetry as the highest occupied molecular orbital (HOMO) of the product (a butadiene). This can only happen with a conrotatory ring-opening that results in opposite signs for the two lobes at the broken ends of the ring. (A disrotatory ring-opening would form an anti-bond.) The following diagram shows this: