Constantius died at Eboracum (York) in Britain during a campaign against the warlike tribes of the North on 25 July 306. Just before he died, Constantius conferred Imperium on his son, Flavius Valerius Constantinus (later Constantine the Great). The army commanded by Constantius wanted Constantine proclaimed Augustus to succeed his father, however, Galerius Maximian, the now de-facto senior Augustus elevated Severus to Augustus of the West (in accordance with pecedent) and affirmed Constantine as Caesar (of the West?).
I believe that the Tetrarchy began to fall apart at this point in time. Surely Severus should automatically have assumed the position/title of Augustus of the West (otherwise what would have been his position/title?) and then selected his own Caesar in accordance with precedent. Of course, Constantius conferring Imperium on his son and Galerius Maximian subsequently affirming him as Caesar threw a monkey wrench in the works and Galerius Maximian was stuck with the task of sorting it all out.