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: