Effect of pressure on the conrotatory opening of cyclobutenes. An unexpected result?

Article abstract of DOI:10.1016/S0040-4020(98)83012-3

Against intuitive expectation, conrotatory opening of cyclobutenes 1-4 is slightly promoted by pressure with activation volumes around -2 to -3 cm³.mol^-1. The volume profile shows a minimum for the transition state TS. This result together with low activation entropy values demonstrates full concertedness and suggest a cyclic TS with a preferential orbital topology.