From GT New Horizons

Explosions are an unfortunate consequence of improperly handled technology. Almost all GT machines can explode if exposed to certain environmental factors or actions. Other multiblocks and a few other special blocks can also detonate under the right circumstances (IndustrialCraft 2, TecTech, and other GT child-mods). Machine explosions can cause instant severe Pollution in their chunk. Explosions and their causes are recorded in your_instance/logs/Explosion.log

Cause Explanation
Active Transformer Attempting to modify any part of an Active Transformer multiblock while it's powered will explode, so will a nearby explosion.
Amperage Every GT wire/cable is rated for a certain number of amps (1x, 2x, 4x, etc.) Exceeding the amperage rating of a wire/cable causes it to catch fire, vaporize and potentially explode connected machines when power is drawn.
Bed Attempting to use a bed in the Nether, End, Personal Dimension and most other dimensions will cause an explosion.
Elemental Matter TecTech machines that use Elemental Matter will explode if oversupplied. This includes the Decay Generator, Essentia Dequantizer, Essentia Quantizer, Matter Junction, Matter Quantizer and Matter Dequantizer.
Fire Adjacent fires can cause GregTech and IC2 machines to explode. Mob-proof machinery areas and avoid open flames.
Fluid Pipes Fluid Pipes can explode if their heat rating is exceeded. This can catch nearby machines on fire.
iTNT Industrial TNT is primed and will always explode if interacted with.
LHE / Boiler Adding water to a boiler or Large Heat Exchanger that is heated past 100C will cause an explosion. Dry with no water added or heated but with consistent water is safe. Advanced Boilers will not explode if they run dry.
Lightning Fires can explode machines; don't keep them outside unprotected.
Lootgames Failing a Lootgames puzzle with no successes may cause multiple explosions. Failing a level of minesweeper also explodes.
Mobs Creepers & Fire Bats explode on contact, potentially taking out nearby machines. Ghastly Infernal Mobs shoot fireballs, Storm can cause lightning strikes. Skeletons can have explosive arrows. Passive animals also have a small chance to explode if killed without a Butchery Knife or Meat-o-matic.
Plasma Generator Insufficient Energy Dynamo on a Plasma Generator for power produced will cause its Energy Hatch to explode.
Rain Rain makes GregTech and IC2 machines explode if it falls on the top or any orthogonally adjacent block (not diagonals). Uncovered wires (not cables) exposed to rain can also catch fire. Protect machines by placing them indoors or adding a roof above. Covers also protect the covered side against rain explosions.
Reactors If a Nuclear Reactor, Fluid Reactor or Vacuum Reactor gets too hot, it will explode.
Rocketdyne F-1A Engine Insufficient Energy Dynamo on a Rocketdyne F-1A Engine for power produced will cause its Energy Hatch to explode.
TecTech Like transformers, TecTech machines will explode if they are adjusted while powered.
Tesla Tower Part tier mismatch will cause it to explode. Always upgrade all parts at once before powering on.
Transformer Attempting to switch a Transformer's mode (step up/step down) while it is powered will make it explode.
Voltage Every machine has a tier/voltage rating. Feeding power that's higher than a machine's rating causes an instantaneous explosion as soon as it tries to start a recipe.
Warp Thaumcraft Warp can cause explosion noises, but doesn't actually do damage.
Wither The Wither boss does explosion damage on spawn and while attacking, as normal.