Early solar system had plenty of impact events happening. Mars has less gravity than Earth, so it's easier to eject material from there. Picture a comet or aste

Early solar system had plenty of impact events happening. Mars has less gravity than Earth, so it's easier to eject material from there. Picture a comet or asteroid hitting early mars. Most ejecta falls back to Mars, but still tens of thousands of rock fragments are ejected into space, in random directions. Some fraction of them are likely to reach Earth a few years later, potentially carrying any microbial stowaways present in Mars by then.