The definite article is mostly pronounced 'thuh' before a noun beginning with a consonant (thuh chair), and 'thee' in front of a noun beginning with a vowel (thee apple).
Question 1: what is the name of this pronunciation change?
Question 2: many news and sports broadcasters (noticed especially on Irish television) use 'thuh' and 'thee' randomly, with no regard to the following word. Where did this usage originate?