from what i've read, it is from the motor being out of the powerband, and not swallowing the fuel, and or going extremely rich so yes on the velocity......


and it can puddle if the soleniod leaks, and if you run a wet kit without a purge, you will have a bunch of fuel being dumped in with no nitrous, which can also create and extremely rich condition and possible intake backfire...........


but to answer the velocity question, that seems to be the biggest problem, just not enough velocity to swallow all the fuel and boom!