Why are the clocks set back at 2am?

Question

...and not at another time?

Answer 1

From Wikipedia:
"With a few exceptions, switchovers between standard time and DST generally occur in the early hours of a Sunday morning, because doing so then causes less disruption than a change on a weekday would."

In the US, 2 AM was probably picked because it offers the least disruption - most non 24-hour operations are closed (allowing them to switch the clocks on their way out the door), and 24 hour operations are generally not busy at 2 AM, allowing them to perform the switch without impacting their work.

An additional point is that most bars are closed by 2 AM in many jurisdictions - some locations will allow bars to extend closing hours to what would have been 2 AM during the spring change.