From Paul Yaw of Injector Dynamics:
In any port injected system, there is a film of liquid fuel clinging to the walls of the intake runner.
During high vacuum conditions, this fuel is stripped from the surface, and pulled into the combustion chamber.
Because the throttle is closed, there is not enough oxygen to support combustion. The fuel ignites when it comes into contact with the oxygen at the end of the exhaust pipe.
Note that this occurs even with decel fuel cut.
The fuel that clings to the runner wall, and its response during transient conditions was described in detail by an engineer at Ford in 1981.
It is the basis of most current transient fuel correction strategies.
It is known as the X-Tau method, and it is explained clearly on the Megasquirt website.
Be sure to watch the videos listed at the end of the article, or order the SAE papers for a thorough description.