Final answer:
The explosion of interest group activity in the U.S. is caused by external disturbances such as changes in social norms, technology, and political events.
Step-by-step explanation:
According to the textbook authors, the explosion of interest group activity in the United States has been the result of various external factors, or "disturbances," that lead to the formation of new groups. These disturbances can include changes in social norms, environmental factors, or changes in technology. For example, the altering perceptions on the use of cannabis have resulted in groups advocating for its legal use, while groups such as Britain's Extinction Rebellion formed in response to global climate change. Additionally, political events and societal changes such as the 9/11 attacks have contributed to shifts in political beliefs and partisanship, further influencing the proliferation of interest groups.