Most of those examples are of a different phenomenon. When the relative or interrogative qui, quae, quod is used adjectivally together with a noun and a preposition goes with the whole, the preposition is usually placed between the two words (quam ob causam, qua in re, quibus de causis, etc.)
In being postpositive after qui, quae, quod used as a pronoun alone without a noun isn't so usual, as far as I know, though it can happen, as in the first Plautus example there, if the reading is indeed correct (the cross indicates some doubt, I think?).
In my experience, postposition happens mostly (though not exclusively) with disyllabic prepositions and mostly (though not exclusivley) in relative clauses.