The gendarmes, with the big push for holidays are pulling vehicles all over France including those coming out of Belgium and England, for being overloaded. They will check the details of your car or trailer for max load and if you are over, you will be fined and have to unload your vehicle of excess weight.
http://videos.tf1.fr/jt-13h/les-voitures-chameaux-traquees-sur-les-autoroutes-7406492.html
If you cant understand all the french the pictures will give you an idea of what they are doing.

According to the news report it is just the north of the country.

Yes you are right, could be because they start ahead of the middle of the country on the big push, just giving a heads up in case they decide to move it further along.

They will get bored with that after a couple of days.