Just yesterday there was an article about this in El País. The basic idea is that this is an old use of the -era form of the subjunctive that has made a return.
Originally, the -era form was not the Pretérito Imperfecto de Subjuntivo, but the Pretérito Pluscuamperfecto Simple. There are many examples in Medieval literature. This form started being used as subjunctive, even though that tense already existed (the -ese form, saliese), and, little by little, became almost a synonym and the original usage was lost: today, the Pretérito Pluscuamperfecto is a compound form in Spanish, though the simple form remains in Galician or Portuguese.
Nowadays the old use has a revival in journalistic use. It is somewhat frowned upon by many users, but it really has it roots in centuries old usage, so it is hard to complain about it.