I would assume that the cabin crew are trained not to open that rear door if the plane is tilting forward like that, particularly if it's a known issue on some types of equipment, as Jerry mentioned above. However, it may be very difficult for someone inside to really know how much of a tilt is too much unless you opened the door and looked out, especially in a stressful situation.

Unfortunately, I assume that opening the door to check also automatically deploys the slide whether it's safe to exit that way or not.

In a panic, I could imagine passengers making an unwise beeline for any exit that they think leads outside. Throw in some smoke or flames or the sight of a bloody passenger or two inside the cabin and I could easily see people tumbling out that back door in a panic if the door is open like that unless a crew member is blocking it and turning people back.