What causes a blowout?
Blowouts occur when the reservoir
pressure of natural 
gas or oil is greater than the place
that the natural gas and oil is escaping to.
Reservoir pressure from the
formation of the well is caused due to
hydrocarbons being trapped in a formation by earth
gravitational pressure compressing the natural gas or oil. This causes the hydrocarbons to push to
lower pressure areas.
http://www.mssa.org.my/site/index.php?option=com_content&view=article&id=107&Itemid=168
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So, when drilling a well and
reaching the reservoir of natural gas or oil, the formation fluid would have a
large amount of pressure on it causing it to want
to reach equilibrium. When
drilling from the surface, a place with lower
pressure, to the reservoir, a place with higher
pressure, the formation fluid wants to be sent to
that lower pressure area. Sometimes their is a
large enough gradient in
surface pressure to the reservoir to launch fluids
over 200 feet. Blowouts can also occur underground
with the formation fluid transferring to a
formation with lower pressure. Though it doesn't
escape to the surface the reservoir leaves its
formation to a lower pressure area.
http://www.energyindustryphotos.com/oilfield_blowout_photos_and_rig.htm
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