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.