an accident on the northern carriageway and everyone on the southern had to slow down
This specific example seems quite logical to me, especially if the motorway is busy. If you see an accident happen on the other side it's only natural and common sense to hit the brakes in case some rolling vehicle/bouncing wheel/lorry load or even loose person[*] crosses the central reservation and comes at you.
The braking propagates backwards as everyone behind hits their brakes to avoid a pile-up, and before long the carriageway crystallises into a solid mass. Vehicles near to the front can't get away until those right at the front move on, and it's frustrating but there you are. While you're inching forwards it's a good idea, to my mind, not just to stare fixedly at the brake lights of the car in front but to keep aware of the total environment.
Plus, clocking the carnage caused by someone else's accident might give reckless drivers pause for thought about their own behaviour. (This is not, of course, to condone driving ahead while not looking where you're going, as in the OP's example).
And it's not only accidents that cause rubbernecking... there's a TV clip in which an official is being interviewed about safety at a roadworks, and in the background a driver clearly distracted by the TV crew shunts into the car in front!
[*] An old schoolfriend of mine told how he'd been involved in an accident on the M62; in a daze he'd got out of his car and crossed the central reservation and the three lanes of traffic coming the other way, and then turned round and walked back again, looking for his briefcase!