Why was Agrippa Postumus considered a possible rival to Tiberius?

Agrippa Postumus was considered a potential rival to Tiberius because he was a grandson of Augustus and had been named a coheir to the emperor.1

As the son of Julia and Marcus Agrippa, Postumus was part of the imperial family line that Augustus initially favored for succession. While his brothers, Gaius Caesar and Lucius Caesar, were early candidates to succeed Augustus, their premature deaths left Postumus as a remaining descendant.12

Although historical accounts describe Postumus as a boor who eventually fell into disfavor with Augustus—leading to his banishment—his status as a designated heir made him a political threat to Tiberius's position as the successor. Following the death of Augustus in 14 ce, Postumus was put to death, an act that may have been ordered by Augustus himself before he died, though the certainty of this remains unclear.12