。
She put the result down to her own carelessness.
Literal
She [topic-は] that result [object-を] herself's carelessness [to-に] connected.
結び付ける ('to tie / link / connect') is the same verb that ties knots and forms bonds — applied figuratively here to causally link a result to a cause. The structure ~を~に結び付ける ('tie X to Y') exposes a clean pattern: を marks the thing being attached, に marks what it gets attached to. Where English uses the Latinate 'attribute X to Y', Japanese reaches for a vivid physical metaphor — 'tying' two things together. 不注意 ('carelessness') is a useful 不- prefix word; the same pattern gives you 不安 ('anxiety'), 不可能 ('impossible'), 不公平 ('unfair') — 不 negates the noun that follows.