Why does decompression of the mantle cause production of mafic magma?

Answer 1

Decompression of the mantle produces magma, which is just molten rock. As the upper mantle (asthenosphere) moves closer to Earth's surface, it is under reduced pressure due to reduced depth. It does not get any warmer (in fact, it probably cools) but it melts.
The only reason the rest of the mantle is not molten is the pressure on it; solid rock is more dense than liquid rock, so even when it's above its melting point, it can be pressurized into a solid. Drop the pressure, and it expands into a liquid.

The same thing happens inside an auto radiator; pressure keeps the liquid coolant from expanding and turning into a gas (which does not absorb or transfer thermal energy from the engine well and will damage an engine). If you don't have a tight radiator cap or if you have a big leak in the system, your coolant easily boils.

The opposite happens with water; it expands to form a solid unlike most other materials. If you put pressure on it, you can push it back into a liquid even when it's below 0 degrees C. This is why you slip on ice, ice skates work well, and old snow sleds have just two thin metal runners. High pressure melts the ice and you slide on a thin layer of water.

Why is the magma mafic? It's due to content; being of low silica content it is low viscosity (runny) and will remain that way unless it passes through continental (felsic) lithosphere and picks up silica, thickening as it goes.