There is a long history related to the ethnic origins of the conflict in Ethiopia. Eritrea was part of Ethiopia until it achieved its independence. It used to be the province just north of Tigray. There's obviously, from what we can gather, communication and coordination between the Government of Ethiopia and the Government of Eritrea, enabling it to enter the war.
When the Tigray People's Liberation Front led the Government of Ethiopia, that was when the war took place. Now, of course, Eritrea is an ally, if you will, of the Government of Ethiopia. It's a simple story, but it's a story that has roots that go back to ethnicity and conflict over the last 20 years.