In an article on geology.com republished from The University of Wisconsin-Madison by Jill Sakai, Sakai discusses what may have been responsible for the reawakening of Iceland's Eyjafjallajökull volcano. In a study cited from The Journal of Nature, it is believed that the volcano may have erupted from flowing magma beneath the volcano. It was also found that the volcano swelled from the formation of magma for 11 weeks before it erupted. There are records of an increase of earthquakes from agitation in the mountain as the date of eruption came nearer. Once it had erupted, the volcano maintained its relative size and amount of magma unlike most volcanos that deflate. The volcano then erupted a second time, this time from another ice covered vent. The Eyjafjallajökull volcano is a great example of how an unexpected volcano eruption can prove to be catastrophic. If the a similar incident had been close to a population then many people could have been killed.

http://geology.com/press-release/iceland-volcano-monitoring/
http://www.natural-calamity.info/the-volcano-under-eyjafjallajokull-glacier-eruption-again/