This theory suggests that language changes by contact with other languages or dialects. However, this theory implies that one language is 'lesser' than the other. This theory is typically reinforced by words in texts known to have come from a language whose speakers were forced into a subordinate social position by English speakers. One such example is the influence of Yiddish speaking immigrants in downtown New York City that lead to the formation of the classic inner city NYC drawling accent, with pronunciations like 'kaw-fee' for 'coffee'. Another is the accretion of loan words by English from language spoken in its previous imperial colonies, such as Hindi borrowings like 'veranda' and 'bungalow'
RANDOM FLUCTUATION THEORY
This theory is relatively simple, and states that language changes thanks to random mistakes and fluctuations by its speakers. One such example is the birth of the word 'goodbye', which came from originally from the phrase 'god be with ye' contracted into 'god b w ye', then consequently being misread and/or mispelt until 'goodbye' appeared and became standardised.
LEXICAL GAP THEORY
This theory also ties in with functional theory.
According to this theory, language gains new words and terms thanks to their being a convenient 'lexical gap' already existing. A lexical gap is a space in language for a particular word which does not yet exist, but fits exisiting patterns of grammar and syntax. For example, there is a noun for a child who has lost their parents ('orphan'), but not a noun for a parent who has lost their child. This theory of patterns also spawned the idea of phonological gaps, for example, the words lip lap lop exist in English, but not lup or lep which would fit the pattern.
This theory ties in with the lexical gap theory.
A reasonably simple theory which states that language changes because it is necessary. For example, when a durable, waterproof, plastic-based material was created a name was necessary. Hence, the concrete noun 'vinyl'.
JEAN AITCHISON: 'Damp Spoon Syndrome'
Jean Aitchison came up with a series of inventively named theories regarding language change, the first of which was the Damp Spoon Syndrome
This theory states that language change is down to laziness, much in the way someone puts their teaspoon back into the sugar bowl after they finish stirring their tea, not bothering to wipe it clean (hence the theory's name). Aitchison does stress that the only true lazy speech is when the speaker is intoxicated, so muscles cannot function properly causing slurring
JEAN AITCHISON: 'Crumbling Castle View'
Aitchison next came up with the Crumbling Castle View, which stated that English was once at an unspecified perfect 'apex', and any change away from that could only be negative. This view is largely taken by prescriptivists, and they seek to preserve their language like an enormous castle breaking down over time
JEAN AITCHISON: 'Infectious Disease Syndrome'
This theory states that 'bad/poor' language spreads like a disease through a population. Changes catch on in social groups and move fast. If you hadn't already guess, Jean Aitchison isn't unbiased.