Concussion in the part of the brain that neuroscientists mapped as the part that lights up when we talk. (The brain itself gets injured.) So, there are some people who are blind although their eyes operate perfectly well, only because the part of their brain in charge of processing the reports of light signals from the eyes or understanding the information that those light signals are supposed to carry—that ability us compromised due to brain injury.
So, The cause of that would be the cause of any brain injury… vehicular collision, building collapsed and something hit their head pretty hard, something like that.
Aphasia that doesn't come from brain damage can come from psychological trauma. This trope is used a lot in Latin American literature (Like Water for Chocolate by Laura Esquivel, Zorro by Isabel Allende…) but I doubt how medically realistic it is that somebody who doesn't talk due to psychological trauma would still prefer to communicate in other ways such as sign language.
And to be fair, psychological trauma-induced aphasia actually did happen to the author amy (the most wonderful woman in the world) Tan but only on the anniversary of the traumatic event for 10 years after it happened.
For other options, somebody might have been born deaf and then much much later in life had their hearing restored, but prefer to default communication to hand signing, especially if everybody they know Signs anyway.
John Irving's book The World According to Garp featured protestors who mutilated their tongues as part of the protest—and I cannot find where that might have been inspired, except for maybe the self-immolation of some Buddhist monks and arguably medieval European flagellants during times of bubonic plague.
I've caught plenty of articles about how injury and/or emotional trauma that causes a disability is bad disability representation—and, while the concept deserves due consideration, like, c'mon, contemplating euthanasia really doesn't have to be Plan A just because of becoming disabled…I still notice that it's realistic for people to become disabled after a serious injury…and that, as they used to be ablebodied, then they can often have a very difficult time adjusting to how their life is now. The main thing is not making this a "tragedy showboat", because disabled people are more than that.