<Form /> submits when a child <Link /> with an onClick callback gets clicked. Can be prevented by calling preventDefault in the link's click handler.
I believe this is fixed in v3 https://github.com/Shopify/polaris-react/commit/2d48d8e718f6e5b857f64da97d36dbda15fdeb91
I've confirmed that this works with v3 in a codesandbox so going to close this.