Why does my horn going off without me pressing on it?

The horn pad is more than likely deteriorating inside .. This allows the contacts to close which in turn sounds the horn .. You may be able to purchase a used horn pad from local salvage yard - OR - - - You can have an auxiliary (universal) horn button installed in a convenient location .. Then the *horn* circuit can be eliminated from the steering wheel. Keep in mind the air bag is also incorporated inside the horn pad .. However the 'air bag' function should not be affected at all by *horn pad* failure. Qualified auto repair personnel can easily determine IF this is the actual problem AND is also needed to do the repair. This is something I have had some experience with in the past and is likely what is causing the horn problem you are having.