It's just the way that sub 80 IQ people, usually but not always boomers, emphasize important words that they would pronounce emphatically while talking.

(I guess its always boomers if you use the 'boomer is a state of mind' definition: narcissistic, uninformed, misinformed, absurdly overconfident, infantilizing, extremely entitled, dismissive, never wrong about anything... you get the idea.

Moronic assholes.)

As to why they specifically do this style of written expression?

That's actually an interesting linguistic question, at least to me.

I think its very much an intentional choice, not a result of autocorrect.

Though almost none of the people that write this way do so on platforms that support a basic syntax / tag system to modify your font/text... but they are probably too stupid/ignorant to figure out how to use it anyway.

I know it existed before Facebook, on forums, often used by religious extremists and super patriotic types, but its mass proflieration on Facebook by boomers... if I had to guess, they read a bunch of shitty ads and posts that used this style, from the mentioned precuror belligerent reactionary forum posters, and then just started emulating it.

tl:dr; its a way of yell-talking, belligerent idiots often raise their volume and intensity for nearly every word they say, but then if you tell them to stop yelling at you, they'll say they aren't, because in their mind, they could have been even louder, and with every word.