First, to start off, there's one fundamental point. Japanese tense isn't "past" and "present" the way we understand it in English, but rather "perfect" and "imperfect".
"What's the difference?", you might say. The difference is that past/present focuses on whether on not an action occurred at a previous point in the timeline with respect to now. Perfect/imperfect specifies whether or not the action
was complete or not from the given point in time. When you see a "past tense" (actually perfect) verb in Japanese, think "complete (finished) action". When you see a "present tense" (imperfect) one, think "incomplete (ongoing) action".
This is why Japanese tenses can sometimes function in ways that are unintuitive to native English speakers. You can get sentences like:
インターネット通信販売サイト
を利用した詐欺にご注意ください!
You might think, "Why is it past tense? It even hasn't happened yet!" But the idea is, by the time is becomes a case of fraud (i.e. one that you can and should watch out for) the "using" of the net shopping site on the part of the perpetrator has already happened.
The same goes for something like:
沖縄旅行に行く時に買いました。
...which has a similar meaning to 「沖縄旅行に行く
前に買いました。」It suggests that you bought something (i.e. a new bathing suit) before/as you were about to go on the trip.
So, what does that mean for the example you gave?
Here, you can interpret the sentence in question simply as having the nuance that the grudges hadn't finished piling up, but were still in the process of piling up when she vented them all out. (Note too that while you say "she repressed her anger", 積もる isn't a transitive verb meaning "to repress" like the English, but rather an intransitive verb meaning "to accumulate". It's the same verb used to talk about snow accumulating.) It's not that 積もっていた would be
wrong per se, it would just give it a different nuance, i.e. that they'd already piled up and that was that.