Several factors come together to cause diachronic change, meaning, the change of a language or group of languages over time: language contact, locations, time-saving, and different language mediums.
Language contact: Through time, speakers of a certain language will and have come into contact from speakers of a different language. This contact causes a borrowing of words, phrases, and meanings. For instance, the word "kindergarten," in which is commonly referred to as sometimes the first level of education a child receives is actually borrowed from the German language.
Location: When speakers of a language become seperated by space (regions/countries/continents), their dialect changes to accodomate their region. For instance, this explains why the English language has many different dialects (accents and language usage) between England, Ireland, Scotland, the United States, Canada, and various other locales. It's reflected by outside contact in their region, regional events, and economic advancement.
To save time: This factor, in regards to pronunication change, is self-explanatory. Speakers of a certain language will economize and shave off letters, parts of speech, syllables, and several other things to save time and effort. THis is how we get contractions such as "don't" meaning "do not" and "can't" meaning cannot." The English language made up for this by creating the apostrophe, which reprensents something taken out of a word.
Language mediums: Pronunciation also changes because different forms of communication are created, used, and discovered. Some mediums include writing, verbal language, electronic communication, and many others.