The idea is floated by some people that many apostates "fell" because they were ultra-orthodox, and that they were "looking beyond the mark", or that they were seeking things beyond what is needful. There are scriptures that seem to back up this idea. There is the Old Testament injunction against "steadying the ark", in which story a non-Levite stopped the ark of the covenant from crashing to the ground, but was struck dead because he didn't have the priesthood which was required to touch the ark. Another Old Testament story that reinforces the idea of not looking beyond the mark is the story of the brazen serpent. The children of Israel were suffering from poisonous snakes, and if afflicted, only had to look upon the brass serpent that Moses lifted up and they would be cured. Because of the simpleness of the way, many Israelites refused, and as a result, they died.
(A digression, but why does the church seem to look to the Old Testament for so many justifications? I thought Jesus fulfilled the Law, and did away with such practices.)
The straightforward answer to this question was answered in a recent comment here by Abner Doon, addressing a different subject, when he said,
"If you're going claim that prophets guide your organization, and claim that salvation depends on obedience to said prophets, don't be surprised when members take you seriously... "
If an organization like the church has such absolute claims of authority and demands for obedience, there is no defense that someone can be "too orthodox" in their beliefs or adherence to the rules.
Of course, This argument assumes that everyone who loses belief started out as an ultra-orthodox, iron-rod believer. I think that it is a false assumption. In my relatively small and unscientific sample of acquaintances and friends, there is no single overriding attribute or strictness of belief or practice among apostates.
I do know some apostates who were ultra-orthodox believing members. I also know many so-called "Liahona" mormons who became full-fledged apostates. There are jack-mormons who live heathen lifestyles who now no longer believe. There is an entire spectrum of belief and practice of those who finally come to the conclusion that the church is not for them, for whatever reason. In short, I think that the demographic of apostate mormons is orthogonal to orthodoxy in belief or practice.
I think the warning not to "look beyond the mark" is no more than a scriptural paraphrase of "Pay no attention to that man behind the curtain!" that the Great and Powerful Oz once commanded of Dorothy. I think it has just as much authority behind it as well.