Because the European grid usually has less demand, more supply in summer and more demand, less supply in winter. This leads to EDF scheduling most maintenance and refueling for nuclear power plants in summer. This year was particularly bad because the normal summer maintenances were accompanied by 1. a bunch of delayed maintenances that didn't happen due to COVID-19; 2. a potential issue with welds in a safety cooling system which was found in 12 of the more recent reactors and required some unplanned shutdowns to investigate and (sometimes) repair.

For what it's worth:

it took a worldwide pandemic slowing down the rate of schedules maintenance along with a rare, unexpected potential failure along with the consequences of 30 years of not investing in training new people in the nuclear sector for it to happen, and France is still doing just as well/bad as neighbouring countries (actually, Germany with their full renewabls push is doing much worse). It's relatively safe to say that it's not exactly a common thing.