This is called a "Forbush decrease," named after American physicist Scott Forbush who studied cosmic rays in the early 20th century. It happens when a coronal mass ejection (CME) sweeps past Earth and pushes galactic cosmic rays away from our planet. It sounds counterintuitive, but big solar storms can cause sharp decreases in space radiation.
This Forbush decrease is over, but more are in the offing. Solar Max is coming and soon the sun will be hurling many more CMEs in our direction. Their cumulative effect could create a sustained decrease in cosmic radiation, lowering dose rates for astronauts and air travelers. Stay tuned for updates.