This is one of the auto-correct options.
First, open the AutoCorrect Dialog by hitting ALT+F, T, P (which gets you to proofing options)
followed by ALT+A (which opens the actual dialog).
When I open this the first thing I have focus on is the "Replace Text as you type" list with focus, as it so happens, on open parenthesis, C, close parenthesis with the replacement text of the copyright symbol. I have to select it so that this is transferred to the boxes where you type the actual character sequence and replacement text then hit the Delete button. This will remove it from the list of items that are subject to replacement when typed.
A great deal of intelligence can be invested in ignorance when the need for illusion is deep.
~ Saul Bellow, To Jerusalem and Back