Context is so fundamentally important in understanding what is happening. Context also dramatically affects what something like a "declaration of war" by a specific (ethnic) group actually means.
In Burundi, where a decades long civil war that killed around 300 000 people ended in 2009, and where a disputed election in 2010 resulted in a worrying number of opposition leaders going underground, a call by a former rebel group for the president to step down and an announcement that they intent to fight the government militarily is something to worry about. It may well signal a return to hostilities in this tiny central African country. (population approx 8.5m). Tensions have been growing in the country over the past couple years and the actions of donor nations are probably not helping to make things better.
But the fact that a group decides they would like the government to step down and/or declares war on someone or something doesn't necessarily mean the country is about to fall into disarray and violence. In Zambia, for example, one group declaring "war" on another prompts consternation, outrage and determination that the perpetrators will be tracked down, as well as the suspicion that this might be a ploy by the ousted opposition party to discredit the president.
The contrasts are stark. Here's hoping the situation in Burundi doesn't get as bad as is feared.