A kicker is not merely backstory that the GM incorporates into the ongoing fiction.
Here's an example of backstory that the GM incorporates into the ongoing fiction:
Player: My PC was a soldier in the Old Guards regiment, before becoming an adventurer.
<Time passes>
GM: As you ride along the road, you see a group of soldiers marching towards you. From their insignia, it looks like they're the Old Guards.
Player: Cool! Do I see anyone I served with?
But there's no kicker there.
Here's another example of backstory that the GM incorporates into the ongoing fiction:
Player: I wake up in bed. There's a body next to me, apparently dead! I can't remember what I did last night - but I can hear someone pounding at the door. I think it's the police!
GM: They call out "Open up! We've got a search warrant." What do you do?
That looks more like a kicker. What would confirm that it was a kicker would be the ensuing play, in which the questions like
Whose body? and
Why my bed? and
What happened to me last night? and
How did the police know? are at the forefront of the action.