Why is Ethiopia facing civil war?

It will stop.

Ethiopia is ethnically heterogeneous. That's why it used to have an emperor - the majority in Addis Ababa and surrounding area are Ethiopian Orthodox Christians who speak Amharic. There's a large Muslim minority, but they also speak Amharic, and when I've been there, I've seen little evidence of tension between Christians and Muslims. But other regions of the country are inhabited by people speaking different languages, some of which are completely unrelated to Amharic (which is a Semitic language related to Hebrew and Arabic).

There was an attempted coup in Ethiopia during my last visit in 2019. My colleagues and I noticed nothing - a few trucks with soldiers, and the internet was down. But we were in Addis; the whole thing happened in the Amhara region, and the effort was to overthrow regional rather than national officials. Ultimately, what's happening in Ethiopia is efforts by different regions to become fully autonomous or independent.

This is, in essence, what led to the restoration of an independent Eritrea in 1993, which had been part of Ethiopia (formally or de facto) since WW2.