I guess she's talking about those people who view Atkins as a weight loss tool, but they intend on eating "normally" (that is the perception of what is "healthy" i.e. more rice less fat) when they hit goal weight.

Soon as they trade the nuts for the refined grains, even if they intended to be moderate, they find themselves eating more and more because they are hungrier. Than before you know it, bam, they're back at their original weights.

They key to avoiding this slippery slope, of course, is to resist falling pray to the erroneous belief that Atkins is "abnormal" and "unhealthy" and that one should attempt to "eat normally and healthfully" at goal weight. As someone in maintenance I can tell you the pressure is overwhelming to eat more grain and less fat. All the time people are telling you "hey you're thin now! Why are you still "dieting"! Eat like a normal person! Some pasta won't kill you!"
I often find myself giving in not to temptation of foods (I have very good control), but to societal pressure. If you give in, even if you intend to be very "moderate", you run the risk of starting on a downward spiral which leads to insatiable cravings, appetite, eating, and weight gain.

So I can definitely see how it can happen to people even if they never intended to "go off". Once you're in the grips of carbs (and they can get you even if you never intended to saddle up a plate of cookies & cake), it's like being in the grips of addiction... it's very hard to pull yourself form that. Like with addiction, it usually takes hitting "rock bottom" to get you to overcome it. Unfortunately, rock bottom for most people winds up being surpassing their old high weight. Relatively few people can prevent falling off the wagon once they are at goal weight, without it having rolled over them first. In order to prevent it you have to be aware of the cumulative nature of addiction. You have to consciously be aware of the fact that addiction overwhelms your conscious mind and your ability to trust your perceptions of reality. Even if you just have 1 now, eventually you will want 2, and without even realizing it you one day wake up to discover you've totally lost all control and are heavier than you were before.