I am certain that another reason for the student's engagement was the strong connection of the course with their community project. They care about their partners and their project's success.
What happened is that the students left hungry for more content. Most of them asked if they could attend the course again this quarter, without receiving credit, just because they wanted to learn more.
What if at the end of every course the students want more? What if they didn't see the class as a box to be checked, but as something interesting? What if we stopped cramming stuff down their throat in order for them to regurgitate the content on an exam, and gave them tasty morsels that they will seek out in the future? This is my yardstick now: When the course is over, are the students hungry for more?