A lot of Newgrounds voice actor reels have what I'm assuming OP is referring to. It's a kind of self-consciousness that isn't yet mastery of the voice to sound realistic, but strained and forced, like the VA's prioritising speaking clearly before expression and doesn't have the talent for both. They have aspirations so feel the pressure to perform. Ironically, I far prefer utterly amateur voices from people who don't intend to be VA's because that self-consciousness isn't there; it's stilted and unrealistic, but it's human.
I would blame VA's having their standard set by mid 2000s web cartoons whose VA's who were raised on 4Kids dubs, or even just Egoraptor (wow his mic sounds good and shouting is funny!). Hence the comparison to Newgrounds reels. Unless OP's "theatre kid" is some hair-splitting other sub-sub-genre of obnoxious inauthentic acting, in which case I don't get it.
https://youtube.com/watch?v=_sjZJ9i_mH0&t=3m20s