UK emphasis on the second syllable vs US emphasis on the first

I can’t explain an unknown individual’s reasons for pronouncing words a certain way.

/kənˈtrɒvəsi/ is a common pronunciation in the UK, though as noted by Oxford it’s “widely held to be incorrect”.

/jəˈhuː/ may be different from your expectations but again listed by Oxford as an accepted pronunciation, so hardly “unbelievable”.

It sounds like you’re over-generalising from a single experience. I’ve certainly never heard /ɛmˈfasɪs/ or /sɪˈlab(ə)l/.

Addendum 1: As a counter-example, I heard a British TV announcer the other day incorrectly pronounce a word that should have the stress on the second syllable: incomparable as /ɪnkəmˈparəb(ə)l/ instead of /ɪnˈkɒmp(ə)rəb(ə)l/.

Addendum 2: A different counter-example – where American English puts the stress on the second syllable while British English does not – is altimeter (AmE /alˈtimitər/ vs. BrE /ˈaltɪmiːtə/).

(BrE) Both Cambridge and Oxford Advanced Learner's dictionaries allow for both pronunciations. It's long been a joke in the UK that it's controversial how you pronounce controversy.

I seem to remember reading somewhere, probably in David Crystal's 'Stories of English', that the favoured pronunciation used to be on the first syllable, then switched to the second. And for me stressing the first syllable now seems a little strange. But I don't think this is a particularly general rule.

I've noticed for example, with that words that have come from French (probably more recently), such as ballet, where we Brits usually put the stress on the first syllable, Americans often stress them on the second, probably getting closer to the equal stressing of the original French.

For the same reason some British people pronounce "toast" almost so that it sounds like "taste" to my American ears. And why they call the hood of a car the bonnet to boot (oh, wait, boot is the trunk).

Also for the same reason that people in this southern U.S. put the accent on the first syllable on insurance.

These are just examples of variations on English. There are many that you will find among all speakers of English.