Why do some British speakers of English emphasize the second syllable of words such as con-TRO-versy. One British woman I knew (living in Oxford) did this to many words including (unbelievably) the search engine yuh-HOO.
I had never heard anyone (Yank or Brit) put the em-PHA-sis on the second sy-LAB-le quite as much as she did.
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/).