In this case, Trigger is a verb. A cause to happen. It is like activate, bring about, cause, elicit, generate, give rise to, produce, prompt, provoke, set in motion, set off, spark, start an Idea.

To cause an idea happen.

If you trigger something you start something Like a chain reaction So if you trigger an idea you get an idea that has came from something So if someone did something, that something could trigger an idea :)