You could also see it as limiting (and controlling) the production of plant maintenance work. For an example of why this is anti-competitive imagine there are

You could also see it as limiting (and controlling) the production of plant maintenance work. For an example of why this is anti-competitive imagine there are 4 producers A, B, C and D. A, B and C agree that they will co-ordinate shutdowns such that they are shut down for four months of the year each. D is not party to this agreement so must shutdown at a time when one of the other producers is shutdown. During this time their are only 2 producers online meaning increased prices and a net benefit to A, B and C whilst the cost of getting maintenance engineers will also increase because they are working simultaneously with either A, B or C which is a net loss to D.