Why is the term "depressed" often used to describe a button which is pressed?

From TheFreeDictionary.com:

depression 1. a. The act of depressing. b. The condition of being depressed.

And the verb:

depress 4. to press or push down

So ... depressed works just fine for the state of a button being pushed in.

"De" is also used as a prefix meaning "down to the bottom" or "away", which can also lead to "completely", as in the examples here: denude, denigrate.

It may count as an auto-antonym, also called contronym. But I can't think of an example of un-pressing something.

Not sure, but my guess is that it's because technical writers are trying to make a distinction between the act of pressing something and the state of something being down/pressed/depressed.

depress and press are pretty much exact synonyms, but almost no one ever uses the term "depress" to describe the act of pressing something. It's correct, but the usage is rare. It's usually used to refer to the state of something. The word pressed is used both ways. But technical writers exploit the rarity of the usage of "depressed" to emphasize the distinction between between the state-word and the action-word.

So:

He pressed the button, and it was pressed.

Was the button actually in a down-state after it was pressed, or is the sentence just saying the same thing twice, that he attempted to press it?

He pressed the button, and it was depressed.

Okay, got it, his attempt to press the button was successful, and it left the button in a down-state.

Depressed can also mean the the button is in a lowered state, which is the result of pressing it.