Why such a huge number of French civilians killed on Normandy’s D-Day?
Probably the biggest contributing factor was the intense bombing campaign that accompanied the invasion.
Carpet-bombing was conducted on the battlefronts -- problem being there was a strong tendency for the bomb-line to "migrate".
Wehrmacht camps and Luftwaffe AA installations were also targeted, along with RR yards and bridges. Many of these sites were adjacent to residential areas. Bombing accuracy at the time was nowhere near what it is now -- especially if it was the RAF (who didn't have the Norden bombsight).
Think about how much naval artillery and bombing went on prior to and on the day of the invasion. Many of the Axis troops were in hardened concrete bunkers, whereas the civilians were in their homes, fields, shops and factories.
My guess is the crossfire - artillery, naval bombardment, and mortars. There were also a lot of French Resistance involved on D-Day, but no where near casualty count.