I don't know the specificities of this particular transition but in general military "rebellions" with legions supporting their direct superior to the imperial throne was a common situation. In some ways it could be seen as an inheritance of the civil wars of the Ist century BC that lead to the principate. All the commanders of the civil wars were famous leader wich enabled them to recruit troops. Even if the situation was a bit different in the imperial era, it's not really different. Basically the senate had no real influence in the nomination of the successor, in particular after the death of a weak/despised emperor. Look at the succession of Nero, the year of three emperors. One notable exception would be the nomination of Nerva after the assassination of Domnitian. One key element in those transition would be to secure key military figures especially the praetorians. Nero was the first to make a gift to these soldiers to secure his accession after the poisoning of Claudius. So this problem was not new at all and would lead to much more destructive civil wars in the second part of the third century. Ironically the senate was theorically the institution that gave the imperial powers by law until the very end (as showed by the lex de imperio Vespasiani) that's why it had the power to appoint a new emperor under gothic supervision after the city was taken by Alaric in 410.