Shortening of capslock, itself a shortening of "capital letters
lock". As the usage of capital letters online often carries the connotation of yelling or screaming (which can be rude), leaving capslock on accidentally can lead to confrontation, hence "caps" to inform people that you did it by
accident. I have no clue why this
shit has not been added yet considering how important it is to internet etiquette.
The phrase "caps" itself can be considered a shortening of "sorry, left capslock on". "sorry, caps" is also common.