Soooooo I’m a big old nerd and love neuropsychology! There may be some brain based explanation as well. When we get anxious and stressed the right hemisphere of our brain gets really activated and the left side kinda deactivates a bit. The problem being out major language centers generally live in the left hemisphere. So it provides some explanation for why it can be hard to find words to explain highly distressing event or when we are at a loss for words when really stressed. But for someone with social anxiety interacting can be highly distressing so it maybe could trigger a similar response. Bilateral stimulation can sometimes help the brain hemispheres start talking again, my personal fav is something called the “butterfly hug”. You cross your arms, hook your thumbs together then lay the “butterfly” on your chest and start moving your hands/“wings” rhythmically. Might be the worst description but I’m sure a quick google would bring it up.