Hi,
I find the sky continues to darker after the end of astronomical twilight when observing in Topsfield, MA, USA. I would put this down to the turning off of lights over the region.
By contrast when in Rush, Ireland I do not see this sharp decline. There, lights are no longer turned off.
An interesting article, Some Thoughts on Twilight, from Sky & Telescope (Oct 1960 p.207) by Joseph Ashbrook talks about this subject. I believe this can be found at archive.org.
John