It happens for various reasons really - sometimes a frequency clashes with another, not pre-thought, allocation to another airfield or agency. A case in point of this is 133.4MHz - currently used by the TRENT sector - which suffers from dreadful intereference sometimes from aircraft calling a French ACC sector.
As a result of this that frequency will be changing soon too - funnily enough to the old approach frequency - 119.525MHz.
Other times it's just as a result of reorganisation of equipment and transmitter sites.