I saw a post on tumblr talking about situational mutism and I reblogged it with a long ass breakdown of my general experience and knowledge, and I wanted to put it here too cause I feel like it's important.
As someone who was surprised the first time a psychotherapist told me selective mutism is a thing and that I have it, here's some stuff I've learned over the years (since a lot of people seemingly still have never heard of it or know anything about it):
Selective mutism has a lot of overlap between autism and anxiety disorders! People with autism can be well known for "shutting down" and going non-verbal, especially when things are overwhelming.* This is the same for anxiety. Selective mutism is in of itself an anxiety disorder and is a defence mechanism akin to freezing during fight, flight etc. The silence is an attempt to protect you as the assumed best way to avoid conflict.
In my experience it feels like my mind is going a mile a minute, just filled with all the things I want to say and how I feel about the current situation but my body is stuck, and no matter how hard I try nothing comes out. I may want to speak desperately but it just doesn't happen. Being mocked for this or having angry reactions and forced attempts to make me speak do NOT help the situation at all. It makes me retreat further into myself. This ties in nicely to my next point.
People who go mute/non-verbal are not toxic for doing so. They are not purposefully trying to hurt those around them. They are not trying to ignore you. They are not giving you "the silent treatment." They can't help it and it doesn't make them feel any better than it makes you feel. Just give them space or comfort or whatever they need to recover and feel better.
Additionally, if you surround yourself with a positive environment and a good support network, then it can help manage or alleviate the symptoms, which is likely why some people may find themselves not going mute around those they trust and are comfortable with.
*I haven't been diagnosed with autism or adhd or anything like that but my close peers (some of which are formally diagnosed) and I believe I am and I'm working on getting an assessment. So if I said something wrong I apologise.