Nitrogen tetroxide and monomethyl hydrazine are "hypergolic" propellants, meaning they ignite spontaneously when they come in contact with each other, a useful property for maneuvering systems.
What is more, most of the 13 tonnes is made up by the propellants unsymmetrical dimethylhydrazine (UDMH) and dinitrogen tetroxide (DTO), both of which are toxic.
The propellants, unsymmetrical dimethylhydrazine (UDMH) and dinitrogen tetroxide (DTO), are highly toxic but they will almost certainly be consumed in the fireball that engulfs Phobos-Grunt when it makes its death dive to Earth.