A Chaos Effect has a chance to occur when one of the following conditions has been met
A Chaos Mage cast's a spell
A follower of a Chaotic deity calls upon divine power
A Chaos Psionist unleashes their mind powers.
A spell is cast in Wild/Chaos zone
A Chaos artifact or item is used. This can effect non-magic users as well.
A curse has been placed on the subject
The chance of a surge occuring is roughly 1 in 20. Or to put it another way, roll a D20 when a condition has been met. You get a 20, you got an effect.