What is the most common way an invasive species is introduced into an environment?

It depends on the species, but 1 common theme is that they are usually initially introduced by humans.

Aquatic non-native species can be introduced to estuaries (green crab, zebra mussel) by ballast water in big ships or on boat rotors and wader boots between streams and lakes (New Zealand mud snail). Terrestrial species are many times transported by direct trade, as with ornamentals (Japanese knotweed), agriculture (grasses), and cultural goods (Kudzu).

Once these species are released into the wild, they often have many new ways of getting around. Human transportation networks, such as roads and power right-of-ways, or hitching rides on wild animal fur, allow further penetration into natural areas. Even on the boots of the nature lover on a wilderness trail.