Why is it common for people to default to a single causal source to explain new phenomena?
I don't quite know an agreed upon word for this type of thing, but I do know what you're talking about. 

At a very basic level, we use previous beliefs to interpret new information. In a way, it's at the core of how we perceive at all. I don't necessarily see these types of fallacious thinking as exceptions. Their belief about the order of the world is different, and so they interpret individual events differently. The examples you listed seem like instances of this happening. 

If anything, it's just simplistic thinking, or a tendency to over-synthesize information. Illuminati type conspirators need to bring a bunch of disparate information into a model that makes sense, so they form connections between things that don't really connect. It probably doesn't help that they expose themselves to a lot of information presented in a way that strongly suggests some sort of Illuminati presence.

But don't forget that we do the same thing. Many times, the difference is that those around us also agree with the assumptions we make, so we don't even notice.