Is there any reason why the surprise can't involve someone or something they've already encountered during the campaign?
There's at least one - The PCs have not made a lasting or strong connection to a suitable element in the campaign to motivate them to risk their lives.
There's another - the player *likes* having their history used as a motivator.
Case in point, from my own game:
The PCs are in Dodge City, Kansas, and they learn that out on the plains, something is attacking and killing buffalo hunters and cattle. They ignore this plot hook for months in-game time, preferring to hunt down stagecoach and bank robbers, which is okay by me.
One of the PCs sets himself up working for one of the local businesses that manages distribution of cattle out of Dodge. Some people he's met at work are killed by aforementioned thing. The party doesn't move on the plot, preferring to involve themselves with some espionage linked to the family of one of the PCs, which is also okay by me.
The PCs discover that dealing with whatever is happening on the plains might prevent a major catastrophe for a childhood friend of another of the PCs - now they are all over the problem like fruit flies on an overripe plum!
I understand that some folks like to play without consideration of character history. One of the PCs in my game is much like this - his character backstory is minimal, and that's fine. But many also like to play that the person and personality they're portraying comes from somewhere, and that somewhere is important. There are pivotal events and people back there.
I've got six players at the table, and I run four-hour sessions. Bonding between the PCs is more likely to happen than PC/NPC bonding. The players each have to worry about portraying one character - I have legions. Each of their characters gets a full four hours a session, while each of my NPCs gets minutes. So, their portrayals are going to be richer than mine, in general, and the interactions between them are usually going to be more interesting than interactions with me.
So, it makes sense to make at least occasional use of the pivotal events and figures that are of interest to one character, and let their internal bonds pull them to action, rather than to exclusively use the generally weaker connections they forge after the game begins.