Horn randomly honking???
All joking aside, it sounds like you have a loose or frayed wire if it only happens when the vehicle's weight shifts. Start by checking the wire that connects to the horn.
I did have this happen to me once in my first car, a grand marquis. A horn wire metal connector had came loose and rattled around the inside of the steering wheel. Occasionally it touched the other connector completing the circuit. Hilarity ensues.






I'm sorry man... I just can't stop picturing that in my head. LMAO 