In a general psych class the professor said that it is kind of like the front of your brain is short-circuting and thoughts then get repeated over and over again until the increased activity in that part of the brain calms down. She said anxiety triggers that short-circut and that that is where obsessive thought comes from. She said nobody knows for sure what causes some people's brains to have that reaction to anxiety while others don't but like said above it may have to do with the neurotransmitter seratonin properly connecting to the receptor nerves.