The why is not really important to the movie. The only mention of why is the president taking a third term and possibly also doing airstrikes on citizens before the start of the civil war. The film more focuses on the how of a possible civil war: the societal breakdown, the atrocities, how people are making it through, etc. There is not really a clearly good militant group in the movie and it is not clear that things are going to somehow be all good if/once the president is removed from office.

I think there were definitely some things that could have been added in like many more corrupt as fuck checkpoints blocking roads. They did show a checkpoint but the characters had no issues with it. Checkpoints are a very easy way for soldiers or locals to shake people down.

If you want something that goes more into why a civil war might happen, I recommend the beginning of the podcast series It Could Happen Here. It was made back in 2019 and foretold some of how things unfolded in 2020 and 2021.