Yeah, mine started recently and I don't know if I induced it or if it's just a bunch of coincidences. I'm hopeless either way. At least, no more intrusive thoughts.
As far as I know, there are many reasons for it, the ones I remember are ;
Allergies, biochemical problems, atlas misalignment, autoimmune diseases, infections, reduced blood flow, brain damage (frontal-temporal damage leading to impaired wernicke's and broca's area) and even brain tumor. Psychosis seems to cause it too (there's a depersonalization-derealization disorder also, but I don't know much about it and I'm a skeptic so I'll consider it as a form of brain damage).
There's a meme on 4chan saying that most people are NPCs because they lack internal monologue, although I agree that most people are NPCs, there's actually no proof that most people have impaired internal monologue, quite the opposite actually (it's 20~25% of population that doesn't have it, and many people actually have brain damage so no surprise) but this isn't all that bad, I think it's affects on pattern recognition is negligible (your brain still memorize things for example, even though it doesn't sound like it because you no longer hear the registering of the memories).