What can cause an explosion in peaceful mode?

I was recently relayed an odd episode. My friend was playing Feed the Beast's Monster pack (single player), mining away in the Nether, on peaceful mode, nothing unusual in sight, and suddenly there's an explosion.

Obvious culprits are TNT, exploding machines, and creepers. After verifying there was nothing like that around ("Are you sure you were on peaceful?"), I'm left stumped. I don't know every aspect of every mod in the pack, by far, but this seems odd to me.

What can cause spontaneous explosions in peaceful mode with no player setup?

The fact that your friend was mining in the nether means he was using the "Nether Ores" mod. Nether ores will often have 1 block in the vein act rather like TNT. If you reveal or touch one of these Explosive ores, it will give off a short hiss, similar to TNT/Creeper noise, as well as give off smoke.

If you have a silk touch tool you will be able to mine these explosive ores before they blow, otherwise they will tend to explode in your face and usually wipe out all the non-explosive ores in that vein and set fire to netherrack around the explosion.

Sometimes they explode when you mine them, touch them, step on them, or when exposed to fire, other explosions and on rare cases, spontaneously combust when exposed to air.