What had happened was that Galba, the man who replaced Nero, had grown extremely unpopular and was was being plotted against left and right. In the western provinces, the legions declared a popular general named Vitellius as emperor. But a confidante of Galba named Otho became disgruntled when Galba chose another man to be his successor. Otho revolted with the help of the praetorians and became emperor. He was very popular in the Rome and restored order. But Vitellius thought himself to be the rightful ruler and marched against Otho in a civil war reminiscient of the one fough between Caesar and Pompey. Eventually, Vitellius's general Caecina was victorious, and Otho killed himself.
I don't think that Otho made any serious mistakes which led to his downfall, it's just that he had the bad luck of becoming the emperor when a man who had the backing of nearly all of the West claimed the throne. As emperor, Otho killed his immediate rivals, but made peace with both former Galba supporters and the armies who had been friendly to Nero. If not for Vitellius, I think that he would have had a long, successful reign.