Why do UK English speakers stress certain syllables of words when speaking?

Answer 1

Every word that has more than one syllable has one syllable that receives primary stress. Perhaps what you mean is that some words have a different stress than the English you are accustomed to. Stressed syllables can differ depending on the dialect of English being spoken. I hope this answers your question.

Answer 2

There are regional differences in the way English is spoken throughout the world and even within a single country such as the U.S. These differences can be in word use, pronunciation of words, and even syllable stress. Languages are always changing through time and changes also occur based on who is speaking the language and where they are in the world. Spelling of words varied much more before the printing press was invented. It's possible that word pronunciation may become more similar now that everyone is hearing each other on YouTube and other internet platforms.