When the reason for doing something is... Nothing

Solution 1:

Your definitions are colorful but I don't think exclusive or necessarily correct.

If you cannot think of a real reason you would say that the reason remains elusive. I think fear is a motivation that comes from within, not without. The situation may threaten but it is up to you to be suitably inspired.

Reasons for doing things corresponding to Nothing are usually Habit, Tradition (via Damila) and not making waves. You can go with Habituation if you really need a tion in it.

Solution 2:

What if you do it because that's what you did before, even if the initial reason is now gone?

That could be explained by the word conditioning:

[Merriam-Webster]
2 : a simple form of learning involving the formation, strengthening, or weakening of an association between a stimulus and a response

Ivan Pavlov became famous for his study of classical conditioning, where he trained a dog to salivate whenever a bell was rung at the same time it was given food. Eventually, the sound of the bell alone would cause the dog to salivate—even when there was no food.

It's also called a conditioned response.

Although conditioning doesn't end in tion, it does contain it, so it's at least similar in terms of structure. You might be able to use condition in a certain sentence, but it wouldn't quite fit into the simple one-word list used in the question.