Short answer: most often overfolding, or an oven that’s too hot.
A hollow shell — crisp top with a gap above the base — usually comes down to the batter or the bake. Overfolding duringmacaronage knocks out too much air, so the structure collapses as it cools; fold only to “flowing lava.” Too hot an oven puffs the tops before the insides set, leaving a cavity, so try dropping the temperature a little and baking slightly longer. A stable meringue and a proper rest help too.
Related:raspberry French macarons · glossary:macaronage,French meringue.