You said it already.... its defense. Being vulnerable and having so.eone recognize that vulnerability is terrifying. For whatever reason, the person you were trying to help may have felt afraid. You unexpectedly broke their defenses.

I have ptsd. My therapist put it like this. The trauma lives inside me. I'm its house. Some people will walk by that house and never notice it. Some people will knock on the door. There are some who will break down the door. And a few may try to tear the house down while trauma lives inside. From my perspective, you broke down the door instead of knocking and being let in.
Try a gentler approach. "It must have been difficult for you. We can talk about it if you want to. I'd like to be supportive and help you. You'll need to tell me how I can support you."
Never assume you understand someone else's pain until they explain it to you. Never assume someone is ready to talk about their pain. What works to help you feel better may be overwhelming to someone else.