Final answer:
The growing enrollments in American colleges were triggered by the GI Bill of Rights, which provided financial support for World War II veterans to attend college, and the post-World War II economic boom that led to the rise of the middle class.
Step-by-step explanation:
The growing enrollments in American colleges were triggered by a combination of factors. One major factor was the GI Bill of Rights, which provided financial support for World War II veterans to attend college. This led to a significant increase in the number of students enrolling in colleges across the country. Additionally, the post-World War II economic boom and the rise of the middle class also contributed to the growth in enrollments, as more families had the financial means to send their children to college.