noun - anything, as an act or event, that serves as a stimulus and initiates or precipitates a reaction or series of reactions.

verb (used with object) - to initiate or precipitate (a chain of events, scientific reaction, psychological process, etc.): Their small protest triggered a mass demonstration. to initiate or precipitate (a chain of events, scientific reaction, psychological process, etc.):

verb (used without object) - to become active; activate.

'Triggered' in this context is the past participle of the verb form used without an object.

http://dictionary.reference.com/browse/trigger?s=t

http://en.wikipedia.org/wiki/Participle