why does the water boil at 100degree C when it is under greater than
normal atmospheric pressure?

It doesn't. It boils at an elevated temperature.

Try this site:
http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/vappre.html

http://www.newton.dep.anl.gov/askasci/gen01/gen01712.htm

ON the suface of water molecules posses both potential energy due to
cohesive forces and kinetic energy due to random motion on the surfce.
a system will be stable as long as negative potential energy dominates
kinetic energy
as the temperature increases the k e of water molecules increases and
finallywhen the temperature is 100 celcius ke dominates pe and
atmospheric pressure, vapour pressure  on the surface
as a result water molecules move away 
this is called boiling at 100