Migration, travel, the British Empire and globalis
People take their language and culture with them when they move to different parts of the world. Some of an introduced language is absorbed into the local one, or the introduced language can become dominant in colonised countries.
English has also borrowed words to accomodate new foods and cultural experiences, for example "curry" and "tea".
Globalisation further developed English into a world language. Shopping has become a global business, with designer names and clothing brands from all countries recognisable around the world.
Wars and Invasions
The Norman Conquest and the Germanic tribes who invaded over a thousand years ago had a strong impact on how English developed. Although the UK hasn't been invaded for a long time, the language of warfare has affected the words we use. We wouldn't have words like "collateral damage", "surgical strikes" and talk of an enemy being "neutralised" without the modern lexicon of war.
Language of science and technology
In the 18th and 19th centuries there were many scientific advances, and so neologisms were needed to name the latest of these. Because of the academic prestige of Latin and Greek, many of the new words were formed using these languages, for example "biology", "chloroform", "centigrade" and "claustraphobia".
Sometimes words are recycled and words with higher status are used, such as Latin or Greek words for medical and scientific inventions. BSE (Bovine Spongiform Encephalopathy) sounds much more serious than its colloquial counterpart used by the media: "mad cow disease".
Trade, working practices and new inventions
New words are needed to name inventions and to describe what you can do with them. "Dishwasher" seems a logical choice for the object it names, as does the Macintosh coat for the man who invented it. But "internet" seems more metaphorical, showing that names arise in different ways.
Social, ideological and cultural changes
Changes in attitudes often result in language alterations. When looking at older texts, it is clear that people held different views about certain social groups. As views have changed about the acceptabillity of some language use, English lexis has had to accommodate them. We discriminate less against certain groups within society and in our language use, and are politically correct when talking about ethnicity, gender or sexuality.
Interests in fashion and culture have been consistent and, as tastes change, so does language. It's not just about lexis: cultural change also affects the way we sound, the registers we use and our grammatical choices.