For Marseilles the strike is a result of the mandatory cost reductions imposed by the Eur Commission ( the so called RP2 and PR3 mandatory cost reductions ) in ATM costs and route charges due pressures of the airlines groups..
Consequences were reduction in staff and delaying modernization programs, only 2 of the 5 Centers in France have been modernized so far ( ERATO system) , staff numbers have decreased ( -6%) while traffic is up 8% last 2 years.
As a side note, the strike in Marseilles is from a small union ( but important in Aix-Marseilles center) but causing large disruptions.
In Spain it is very similar . Cost reductions leading to reduction/shortage staff, plus the bulling of the controllers by their employers. Lot of differences en up in court at the moment and the relations management--controllers is terrible there. .
As to the low cost airlines cancelling flights, I understand from insiders that this is a good way to hide their won staff shortages and blame others. Strikes ( or Weather / Cbs in summer) are the usual culprits.
A good example :
we received a text from a certain Irish airline to say that our return flight from Toulouse is cancelled next Sunday due to the ATC strike
Toulouse is in Bordeaux FIR , aflight to/from UK will be not affected by a strike in Marseilles.
Last point , the ATC strikes , unlike the Train SNCF one , have little to do with Macron reforms , the "RP" cost reductions were long before him. , .