Juxtaposition happens in all texts, but sometimes the lack of continuity is more extreme than usual. At its purest (in a translated oriental poem, perhaps) we might be offered 2 one-word sentences. Assumptions readers might make to connect the words include
When we come to a fracture in a longer text (between paragraphs, chapters, etc) we still try to make a connection between the parts. The way we do this will vary according to the type of text we're reading, but typically I suspect we first assume that the text is jumping ahead in time, leaving a gap that will be filled in later. Then perhaps we might think it's a flashback, or a parallel storyline that will be revisited. Only as a last resort do we concede that there may be no causal connection or character continuity.
One of the features of conventional text is that writers have to present things sequentially even if they happen simultaneously or independently. In such situations where narrative breaks down, the terms Montage and Collage become useful. Both describe a non-hierarchical way of incorporating diverse fragments producing a multicentred work. Montage more often concerns assembly using things that have already been "created". Collage is more often used when material of different types is used together and when there's no particular common theme. Gregory Ulmer described collage as "the single most revolutionary formal innovation in artistic representation to occur in our century". This may be because it cuts across the long-cherished Aristotelian notion of organic unity, where each component of a work is a necessary part of a whole. Max Ernst claimed that "Collage is a hypersensitive and rigorously exact instrument, a seismograph capable of registering the exact potentialities of human welfare in every epoch".
In relation to poetry, David Antin remarked "for better or worse, 'modern' poetry in English has been committed to a principle of collage from the outset". With collage in particular, use is made of the difference between the source/material of the fragment and the meaning in the context of the whole - the observer is expected to bob up and down between surface and depth. Flexibility along this dimension is more characteristic of poetry reading than prose reading.
There are likely to be connections between parts, but they may be more to do with surface than meaning - leitmotifs without a plot. In "Thirteen Ways of Looking at a Blackbird" there's a common theme. In "The Waste Land" the links are more tenuous. In other works fragments are only related in that they each mention a red dress, or an accordian, or have someone shouting "Damn". These latter relationships can seem gratuitious, leading to "washing line" pieces (where the only point of the connection is to have somewhere to hang the pieces from) but this is to devalue the surface, which in collage is more relevant than usual. Also such connections make the lack of causal connection more palatable. Sometimes there are traces of a narrative thread in a few of the fragments. There's no narrative impetus or suspense though. Some consider "thematic interplay" the poor man's "conflict and dynamism", a "compare and contrast" task that requires too much from the reader and masks the authorial persona.
The idea of a decentralised network of ideas has been described by Deleuze and Guattari ('rhizomes') but of course goes back much further than that, beyond Wittgenstein's "family resemblances" - "The governing principle of much Persian poetry is circular rather than linear; rather than a logically sequential progression, a poem is seen as a collection of stanzas interlinked by symbol and image - the links being patterns of likeness and unlikeness, of repetition and variation - which 'hover', as it were, around an unspoken centre" (Glyn Pursglove, Acumen 25, p.9)
Parataxis replaces syntaxis - "the dethronement of language and logic forms part of an essentially mystical attitude towards the basis of reality as being too complex and at the same time too unified, too much of one piece, to be validly expressed by the analytical means of orderly syntax and conceptual thought" (Martin Esslin, "The Theatre of the Absurd", 1962.)
On a small scale juxtaposing can happen on a line and can be read as an implicit (though perhaps surreal) simile. "In Surrealist metaphor, two terms are juxtaposed so as to create a third which is more strangely potent than the sum of the parts ... The third term forces an equality of attention onto the originating terms", "Statutes of Liberty" (Geoff Ward, Macmillan, 1993, p. 73-74).
On a larger scale sentences can come alternately from 2 fields - in 'The naming of parts' for example, the reported speech and internal thought alternate. 'Moby Dick' and 'USA' (Dos Passos) inserted non-fictional fragments. Found text can be inserted randomly into a poem, or fragments of different kinds of poems (rhymed and free-form) can be interspliced using a variation of Burroughs' cut-ups technique. Bakhtin's carnival and polymorphism can come into play too.