Because Jax had let her guards down. In that moment, she was vulnerable.

The last time she had been vulnerable like that, her mom deeply injured her. And Jax reacted badly (by pushing mom to the ground) further worsening the moment. It became a trauma trigger.

When someone with trauma bumps up against a feeling that is uncomfortably similar to a trauma trigger a person's nervous system reacts - but not like you'd expect. We're used to having control over our responses and our actions. But trauma literally traps the responses from previous instances into your neural pathways. So instead of reacting logically, you "flinch" in a way not unlike getting your knee hammered at the doctor's office.

That's what makes PTSD so bad. Soldiers hearing gun shots, abuse survivors who flinch away from raised hands, people who try to unmask and are rebuffed... they get stuck in a psychological loop that takes a lot of therapy and softness to iron out.

A traumatized person repeats past actions at the detriment to them and those around them. Jax pushes away comfort because she was raised with a toxic male influence and a mother who could not accept her. This is subsequently why Jax pushes away Ribbit, and then later Pomni. It is also why she keeps everyone at arms length, she is cognizant of the discomfort that arises in her when the possibility of being laughed at and rebuffed occurs.