Here's a possible explanation: Our brains have a vulnerable speech motor system. Speech is a heavily energy dependent process and without the right resources, breakdown easily occurs. This vulnerability relates to deficits in the ability to use energy like glucose. It also relates to blood flow deficits to brain regions responsible for speech. Genetics is possibly what leads to these deficits (this is a good thing). When your by yourself it is a low stakes environment that your brain doesn't require as much resources, but the demand is higher in other situations. When the demand is greater than the supply, breakdown occurs. This is why anxiety is correlated with stuttering, not the cause. Happy to answer any questions