This is a long time coming.
Sudanese Military traditionally have a heavy hand on politics, and this coup is very largely related to the 2019 coup, in it the Military took power and then established a military transitional government, which give way to Transitional Civilian government headed by the Sudanese Armed Force.
Today coup is launched by RSF, against the reigning government, which is dictated by General Abdel Fattah al-Burhan.
There are two reasons for this coup, 1,) RSF wanted power, they were largely left out of Al-Burhan government since 2021, 2.) The transitional government were supposed to ended in 2022, which they don't because Al-Burhan cling on power since then.
There are minimal stuff that happened over the course but the main theme is like this.
3 years ago, and yes, the current one is a military junta.
en.wikipedia.org