Why do some adjectives follow the nouns they modify?

These are called post-positive adjectives:

A postpositive adjective is an adjective that appears after the noun that it modifies. In some languages this is the normal syntax, but in English it is rare, largely confined to archaic or institutional expressions. Aplenty, galore, and the informal extraordinaire are examples of adjectives that are primarily used postpositively in modern English. Name suffixes, such as Junior and Senior, also function as postpositive adjectives modifying proper names.

Why do they follow these nouns? Sometimes it is imperative for them to follow the nouns they modify. For example, in your example, there's a difference between "proper reptiles" and "reptiles proper". Taking a look at another example:

Adjectives may even change meaning depending on whether they precede or follow, as in proper: They live in a proper town (a real town, not a village) vs. They live in the town proper (in the town itself, not in the suburbs)

That's why they are sometimes used after the nouns proper.

This is called hyperbaton, which means to use out of the normal order to emphasis or to modify the meaning of the noun preceding the adjective. Another example is the movie title Mission Impossible. The writing guide Bang: Writing with Impact explains it this way:

Any time you place words out of their normal order, you create impact. You force the reader to pay attention to them, reflect on them, and remember them. In this strategy, you immediately follow the name of a thing with an adjective or descriptive phrase. In this way, the description becomes part of the name of the thing; they are inseparable. When your reader thinks of the thing, he or she will also think of the description because the description becomes part of the name.

Your reader will notice this immediately, which means this strategy can be used to emphasize a key characteristic. However, because this change in word order is so obvious and so prone to sounding contrived, it must be used carefully and infrequently. In the right place, it can be a highly effective technique for emphasizing a point. Use it at the end of a sentence for greatest impact.

“This is a plan impossible.”

The Art of Thinking (1662), or Port Royal Logic gives the example the stars visible where the adjective gives a accidental attribute, as compared with the visible stars, giving an essential attribute.

I've been searching on the Web for the large-ish group of what I've seen called "a-adjectives" that cannot normally precede nouns they modify in English. Examples are adrift, awry, athwart, afraid. There are at least 40 of them. Most of them are NOT derived from French or any non-English language; they come originally from Anglo-Saxon, I believe.

As best I can tell, to use these adjectives before nouns, you have to cange them: a driting boat, a fearful person.

I saw a list a year or so ago, I think on Wikipedia, but I can't find it again.