The tilt-up also comes from improperly treating the saddle leather. When ill-informed riders soak the saddle in oil or anything of that sort, it stretches much more than it normally would. This makes it more comfortable at first, but it quickly goes out of tension. This makes it sag and acquire the incredibly concave shape you see on a lot of poorly-used Brooks saddles. One more reason to use only proofide.

Like jcm says, the saddle will become concave over time. But a properly-tensioned B17 should require only slight deviations from level to keep the sweet spot.