I'll say this for Deano: I'm generally going to disagree with him most of the time, but the majority of attempted explanations I see also come from him. As rebuttals, 90% of them amount to "you're crazy"....which may be true, but isn't a logical rebuttal to the proposed idea.
I can't speak specifically to each conspiracy, but most of the time I feel like it's just massive confirmation bias. We're programmed by evolution to seek patterns and coincidences, which happen all the time. The overwhelming number of people on the planet, multiplied by how many actions we perform daily and thoughts that race through our head (each one a possibility for a coincidence) and sheer probability ensures that even massive coincidences will happen on a regular basis.
This idea applies to both our regular lives as well as the macrocosm of the culture we live in. Anyone looking for patterns will find many of them. And for someone as credulous as most conspiracy theorists, these patterns and coincidences will seem irrefutable, when in fact they are anything but.
Someone made a thread in the OTF about Washington DC being specifically engineered for some nefarious symbolic purpose. There were diagrams, articles, maps, etc. etc. All very impressive looking, and it located an absurd number of patterns. A few were intrigued by it. But then I read through it, and it was obvious that nearly every "coincidence" was simply an arbitrary selection of landmarks, dates, whatever, in order to strengthen the argument. In other words, it was complete crap.
And if you look at most conspiracy threads, it's the same thing. Hand picking evidence from amongst a much (much) larger potential drawing pool that seems to suggest various links between people or organizations. You can do it for any agenda, given enough time and resources (newspaper articles, websites, TV shows, speeches, political policies, etc.). Then they work that evidence into a theory that is "proven" by the evidence, and transcends all of it.
Some may have merit, but the vast majority simply pander to the credulity and paranoia of the public, and feed the need of many conspiracy theorists.