The threat system has been changed as of the latest patches.
Before the updates there was a global threat level and stages would automatically increase in threat from safe (easy) to overrun (very hard) if you don't clear them after a set amount of time. You could also use Karma to increase/decrease the threat level instead of playing or waiting for the timer. This has been replaced w/ a difficulty select instead. Higher difficulties now net higher/exp and item drops but gives a higher stage CD