From other forums (may or may not be true)
Explanation from a BCN ATCO :
Airport was about to change from night configuration to day configuration. At night, runway 02 is used for landing and 07R for take-off, while during the day 25L becomes the take-off runway and 25R is used for landing (unless winds favor runways 07L/R).
Two of the three ground Air Traffic Controllers work in a smaller Tower located near the main Terminal (frequencies 121.65 and 122.225) while the other ground frequency (121.7), delivery and the two tower frequencies (118.1 and 119.1) are located in the main Tower. 121.65 (122.225 not used at night) cleared the Aerolineas Argentinas A340 to cross runway 02, which he thought was not active as he expected the airport to be in day configuration. Meanwhile, the UT Air Boeing 767 was cleared to land on the same runway by Tower (118.1).
Crossing the active runway usually requires some coordination between the two towers but this is not necessary in night configuration.
Source : PilotList, Robert T.