One aspect is that the reaction releases good heat. Good first check.
This doesn't suffice for a reaction to start spontaneously (if this is what you mean, because "spontaneous" has a special meaning in chemistry). For instance a forest burning in the atmosphere releases heat, but it needs a cause to catch fire.
SiH4 starts many reactions spontaneously, for instance it catches fire in the atmosphere at or just above room temperature.
Telling why is more tricky. It needs to find a reaction path where all the energy hurdles are low, and for that, many species must be considered and combined, including free radicals. The weak Si-H bond eases such reaction paths.