Why is pyrite so explosive when heated? I watched this video where fool's gold was heated using a hydrogen torch.  The results were surprisingly explosive.  I was expecting it to just melt into a puddle.  
Is there something special about the pyrite or the hydrogen that caused the explosive actions?
 A: It's an interesting video but I wouldn't describe what's happening as 'explosions', more like rapid deflagrations. Explosions require prior containment (like in a bomb shell) before violent energy release.
What I suspect is happening is the following. When strongly heated Pyrite undergoes an oxidation reaction, reacting with oxygen in the air and/or the torch's effluent:
$$2 \mathrm{FeS_2} + \frac{11}{2}\mathrm{O_2} \to \mathrm{Fe_2O_3} + 4\mathrm{SO_2}$$
This oxidation reaction was used in the early days of sulphuric acid production, by 'roasting' Pyrite in the presence of air because it generates sulphur dioxide gas (a precursor of sulphuric acid).
A hydrogen torch burns extremely hot (maximum about 2,800 C) and will promote this oxidation strongly. Combined with some thermal shock, the release of high temperature $\mathrm{SO_2}$ on heating could explain the Pyrite 'flying apart'. The oxidation reaction is also exothermic (releases heat), possibly further sealing the fate of the Pyrite lumps.
