Maybe you look in the bathroom mirror, or attempt to read a signpost on your way to work. This exposes the illusory nature of the dream and BAM! You wake up.
Some people report having multiple false awakenings in succession, doing the same things over and over, never knowing when they have truly woken up. They keep unconsciously rebooting the waking dream scenario... As uncanny as it sounds, if you have just had one false waking experience, you are much more likely to have another. The conditions are already ripe.