"Detonation" is a little too violent word for this.
There's a "little" difference between a combust of fuel in the closed/pressurized cylinder than, a fuel combusting in open exhaust manifold.
Valve overlapping, high rpms with rapidly lifted/closed throttle can also be a reason for raw unburnt fuel being burnt in exhaust manifold.
I don't know what kind of exhaust manifolds you have seen, but im pretty sure that example, a casted iron manifold doesn't crack from fuel combusting inside it, since it's open from another end. Gaskets might hurt though...