Why did Eyjafjallajkull erupt?

Answer 1

Becasue two seismic plates started to rub together, creating friction causing each plate to melt into each other, creating heat and adding to the magma levels, therefore starting the eruption.

Answer 2

The cause of Eyjafjallajökull's explosive eruption seemed to be the meeting of one body of magma, made up mostly of the common volcanic rock basalt,with another type of magma within the volcano, consisting largely of silica-rich trachyandesite.

The volcano eruppted by slowly drifting over a hot spot, which heats up the magma under the volcano so it starts to rise slowly creating an eruption. After the lava exited out of the main hole it starts to cool. But, it leaves ashes and heavy smoke which humans are unable to breathe in.