Pronouncing the definite article

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?

The reason to have two different pronunciations is to better differentiate the words in normal speech, where there are no pauses between words.

Try saying "the only" quickly a few times.

If you use "thuh", then the sound of the two words runs into each other, and unless you introduce a pause, you get something like "thonly". Pardon what?

If you use "thee only", then the words remain distinct with no pause.

Where there is a consonant at the start of the second word, then this differentiates the words, so the words remain distinct (like in "the tap")

So then, why not use "thee" all the time? I suspect that this is down to muscle movement. Saying "thuh" requires slightly less muscle movement than "thee" and so evolved as the most natural form where "thee" was not needed.