It's a character performing an action; action needs motivation.
"that's what she does" is what Null posted her reason. I think your reason is a good idea, but she had no idea. Again, as a reader, you might not feel it matters, and for all I know, the story as written might sell it strong enough that I wouldn't notice when reading. But if an editor asked and her reason is "that's what she does", that won't be enough. And I imagine that since she's in a professional workshop, she cares about getting published in the future, not just writing for herself. Otherwise there would've been no reason to react like that to Null. I'd even guess that her reaction might have come because she realized she had a story weakness and she became defensive on instinct. It's not that unusual.
What is unusual to me is that she cobbled this together a week before, it's clearly something for the class, not something deeply personal, she would probably not have those kind of weaknesses in something she really cared about, and yet she has this reaction to Null's questioning.
"that's what she does" is what Null posted her reason. I think your reason is a good idea, but she had no idea. Again, as a reader, you might not feel it matters, and for all I know, the story as written might sell it strong enough that I wouldn't notice when reading. But if an editor asked and her reason is "that's what she does", that won't be enough. And I imagine that since she's in a professional workshop, she cares about getting published in the future, not just writing for herself. Otherwise there would've been no reason to react like that to Null. I'd even guess that her reaction might have come because she realized she had a story weakness and she became defensive on instinct. It's not that unusual.
What is unusual to me is that she cobbled this together a week before, it's clearly something for the class, not something deeply personal, she would probably not have those kind of weaknesses in something she really cared about, and yet she has this reaction to Null's questioning.