Ask Mr. Smarty Plants

Actually, some invasives are native, as well. Usually, invasiveness is controlled by the soil, climate or water being wrong for the non-native. However, plants don't necessarily grow where they belong, they grow where they can get away with it. If a non-native plant manages to get seeds in the ground where there are good conditions, it will grow aggressively, shading or crowding out the native plants around it. There is no natural barrier to keep non-natives nor invasives out of a territory previously dominated by native plants.

In the case of Bastard Cabbage, which we are hearing so much about, it is believed the non-native seeds sneaked in as part of an imported grass mix. When the grass seed was put in the ground, probably some of it blew away to contaminate another area right away, and the rest of the seeds grew into plants that quickly grew, flowered and seeded for still more. They grow quickly, are aggressive with long taproots that spread, and very low-growing flowering sprouts that can dodge being mowed. Read the linked article above to learn how that particular non-native invasive got going. The seeds germinate in late Fall and early Winter, and the early rosettes quickly cover the ground, shading out seeds of more desirable native flowers that were there before.