Here are a few reasons why the campaign can be the fault of the player characters...
-They released the BBG from a prison. The BBG may have imprisoned in the past by heroes of old. They couldn’t kill him so they were forced to imprison him in a vault behind traps and monsters. And, of course, the PCs end up releasing the BBG in pursuit of the treasure that must surely be behind all those traps and monsters. Alternatives on this are that the BBG is slumbering/recovering in a special hide-a-away, regaining his strength “until some foolish mortals release me”.
I had an eleven year (real time) campaign kick of when the second adventure the party went exploring into a lost ruin. Defeating the various traps and monsters they finally released a being from a large orb…a powerful necromancer who then continued his plans to subjugate the world.
-They were unkind to the BBG. This can come is two forms. First is that the characters were rude/unkind/mean to someone. This someone could be anyone. Maybe they were rude to the wench at the tavern. Maybe they cheated a merchant out of 2 silver on a purchase. Maybe they used a derogatory term on someone. The point is the characters did something that another person could take offense with.
The second form is that the someone took offense with the characters through no fault of their own but they are still the focus of that person’s rage. Maybe the someone felt the characters were rude (even if they weren’t). Maybe the someone felt the characters were arrogant as they walked down the street. All you need for this is a time and place and an excuse.
The first option is the better of the two. While the second option will work for placing the ire of the BBG onto the player characters, it lacks some weight. If on the other hand, the characters have actually done something unkind to an NPC, even if it was slight, it gives more meaning to the upcoming confrontations.
-They could have prevented the rise of the BBG. Much the same way Spiderman could have stopped Uncle Ben from being killed, the player characters had a chance to stop the BBG from ever gaining power. Of course, to make this work for a long campaign, you have to make sure they fail if they actually attempt to stop the rise of the BBG.
The player character’s may have been in a race to recover a power artifact, but they lost the race and now the other party has it and are using it in their plans to dominate the world. Perhaps the PCs let a minor bad guy go (a minion who spun a story of a pathetic life) and this person rises to become the BBG.
-They provided the BBG with the means to become powerful. This is one of the oldest tricks in rpgs to date. The BBG is in the guise of a merchant/government official/scholar and tasks the PCs to recover an ancient artifact…at which point the BBG reveals that he duped the PCs, uses the ancient artifact to become all-powerful and goes off to begin his plans to conquer the world. While this is a bit clichéd it still works for our purposes. Few things will rise the ire of a PC faster than being duped.
A few of the options above are predicated on a lowly person becoming the BBG after the encounter with the PCs. However, once you have given an antagonist a reason to dislike the player characters, how do you give that person the power to do something about it? It is unlikely that a powerful being will be in a position where the player characters are able to be rude to them (though this could happen if the BBG is in disguise for some reason). The more likely route is to give the future BBG an item or situation that transforms the NPC from lowly-person-oppressed-by-the-player-characters into the BBG. Here are a few ideas for doing this…
-The person gains control of a very powerful being such as a genie or demon. This could be from the discovery of an innocuous item such as a lost bottle.
-The person is an unknown heir to a kingdom and is elevated after the player characters interacted with them. With their new position they will have access to armies, mighty persons, wealth and magic items of high power.
-The person is a scion to a bloodline of powerful sorcerers and their powers manifest later. They can quickly grow from lowly peon to a person wielding potent magic.
-A being (such as a lost angel) or group (such as a cult) is looking for guidance. Due to being in the right place at the right time, the future BBG is able to focus their direction into endeavors of domination…endeavors that the PCs will need to try and stop.
-And the old stand-by of the BBG finding a lost artifact that gives them unlimited power. Again, it’s clichéd but works in a pinch.
The point of all these suggestions is that you want the players to feel as if they were the cause of the problem the world is now facing and that they need to “make things right”. The best outcome is if they even feel as if they could have prevented the problem, but failed to do so. Personal angst is a power motivator.