Contact explosion of nitrogen triiodide
Wikipedia states


  The instability of $\ce{NI3}$ and $\ce{NI3 · NH3}$ can be attributed to the large steric strain caused by the three large iodine atoms being held in close proximity to each other around the relatively tiny nitrogen atom. This results in a very low activation energy for its decomposition, a reaction made even more favorable due to the great stability of $\ce{N2}$.


The steric strain between the atoms can be better visualized by the representations here.

The decomposition reaction is
$$\ce{2NI3\to N2 + 3I2}$$
Note that there is an increase in the number of moles (4 versus 2), which means that the reaction is favored by entropy, because there are more possible microstates.

More specifically, the enthalpy of formation is 154.4 kJ/mol, which is incredibly favorable for a reaction.

Reference: UC Davis ChemWiki