Why does ethene react explosively with fluorine?
The equation gives only the reactants and final products and tells us nothing about the mechanism; stoichiometric equations rarely do.This is an explosive reaction so it will proceed by many steps usually involving radicals. To get feedback, and so explosion, one step must be of the branched chain form $$\ce{R_1^. +  M -&gt; R_2^. + R_3^.}$$ where $\ce{R^.}$ is a radical and $\ce{M}$ a reactant species (e.g. $\ce{CH2=CH2}$). As two radicals are produced for one reactant radical a chain reaction ensues, the reaction rate accelerates and so bang! 

Any explosive reaction has initiation, propagation, branching and termination steps, some also have an inhibition step. The initiation step in your reaction probably forms the radical $\ce{FCH2-CH2^.}$ although I don't know the mechanism.