The higher gear means it wont accelerate as fast meaning more heat will build up in the chamber meaning more likely to have a hot spot and a place for the spark to start at the wrong spot and wrong time. Also the piston will accelerate away from TDC faster meaning a slightly faster than normal flame front could still be chasing the piston instead of coliding with it witch causes more pressure and a faster burn that causes more pressure and a faster burn... long story short it burns so fast it is more like an explosion and damages your engine and causes audible knock.

Exhaust manifolds probably help reduce detonation as they reduce cylinder filling efficency plus there is going to be a slightly higher amount of inert exhaust gas left in the chamber that will slow down the burn of the next air fuel charge.