Why did our manager's closest employee become resentful towards me out of nowhere?

She was always so nice to me. We always talked about girl stuff together, even though she still respected me as a boss. She's such a good girl and a hard worker. She's like the best. However, recently she's been displaying a lot of anger towards me. This just happened out of nowhere. I didn't know what to do. I tried to be as nice as possible. She was never mean to me. She was just overwhelmed.
She never stood up to me in a stressful way or anything. She always went with the flow whenever I told her to do something. Her work performance didn't change. But, I noticed that she's changed as a person. Towards me anyways. This happened after employees took turns having meetings with the store manager. She expressed frustration, fear, and stress towards me. She said how pissed she was and was afraid to get things off her chest. I told her everything was okay. She still wouldn't talk to me.
I tried to calm her down with water. However, I've been very concerned about her. I never thought I would see her act that way. She eventually told me that I was the reason why she was stressed out and angry. She told me that she felt threatened by me for 2 years. I had no idea she felt this way about me. She said I came across as intimidating, mean, and stuck up. She said she didn't feel safe because I tended to lose my cool easily. I try to control that now, but she still fears me snapping at her. She told me personal things that make me feel bad for her. So, I worked with her the best I could. I told her I was sorry and that I'm not that type of person. I always tell her how good she is.
However, she still shows negative feelings towards me. She doesn't hold back anymore. I don't know how to respond to her when she's angry. I want her to go back to being sweet again. Why is she like this?