The problem, at least according to statements from a TV helicopter pilot who was streaming live about 1 hour ago, was that water was running over the area at the end of the emergency spillway and eroding soil there, and that erosion could lead to a failure of the emergency spillway.
They ran out of daylight, but there is video showing the problem area and the traffic backups of people trying to escape Here
Thanks for the link !!
At 0:50 you can see the water going over the weir, —plus— water to the left of it.
That’s what some were worried about, that it would go around, and erode that end.