I've just been googling to try to understand it, and the clearest account I've found is quoted below. It seems to be a matter of extreme right-wing "hooligans" versus more working-class and multi-ethnic "ultras" -- with the government wanting the whole mess to go away for the sake of tourism. But I can't say the battle lines are altogether clear.
http://www.lookleftonline.org/2014/02/how-psg-lost-its-soul/
An old video clip on YouTube shows the players from Paris Saint-Germain (PSG) walking out in front of their home stadium, les Parc des Princes. ... This was the 1990’s, the halcyon days of Parisian fan culture. Supporters groups such as Lutece Falco, Tigris Mystic, and Supras Auteuil competed for visual and vocal dominance in the stands, expressing their devotion, their fanaticism, to a city and club they lived their lives for.
These groups had superseded the less visually impressive and more violent ‘Boulogne Boys’ of the Kop of Boulogne (KOB), at the other end of the stadium. Since the 1980s this terrace had become a meeting place for the Parisian far right and it was a notorious hotbed of racism and fascism, supporters consciously aping the worse elements of English football culture. For many youths from the Parisian suburbs, or Banlieus, the Virage Auteuil was an alternative to this, where predominantly left wing and antiracist groups developed their own form of the ‘Ultra’ culture, which had its roots among football supporters in Italy.
For several seasons these two fan scenes, with their radically different social make-ups, political views, and cultures coexisted in relative peace. Until 2010, when a period of violence between the two groups, which had been instigated by racist attacks by the Boulogne fans on their racially mixed counterparts, led to the death, after a prolonged coma, of ‘Yann L’, a 38 year old Boulogne Hooligan. He had been attacked and badly beaten outside the les Parc des Princes by members of Supras Auteuil.
Following the death of ‘Yann L’ the Club and French authorities conspired to legally ‘dissolve’ all the major PSG fan groups. ... The next step by the club was to introduce random seating – you could no longer choose where you would sit – and the raising of ticket prices. Under the cover of ‘tous PSG’, a heavily marketed program of gentrification of the support, PSG sought, with the help of the Government and media to replace the hard-core supporters with ‘event customers’.
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