One theory is about relative movement of old and young. Normal situation, young heathy people move around a lot more than the elderly, contract the virus get im

One theory is about relative movement of old and young. Normal situation, young heathy people move around a lot more than the elderly, contract the virus get immune. With lockdown everyone is restricted and old and young go to the supermarket and back. As such the elderly are exposed at a higher rate before any level of herd immunity is reached.