You have a good point ASCLEPIADES, infact it's even disputed if there were a conspiracy against Tiberius.
These sources seem to think that it's was Tiberius who conspire against Sejanus
"In addition to his old friends and intimates, he had asked for twenty of the leading men of the State as advisers on public affairs. Of all these he spared hardly two or three; the others he destroyed on one pretext or another, including Aelius Sejanus, whose downfall involved the death of many others. This man he had advanced to the highest power, not so much from regard for him, as that he might through his services and wiles destroy the children of Germanicus and secure the succession for his own grandson, the child of his son Drusus."(Suetonius, Life of Tiberius, 55)
Ann Boddington think that infact Tiberius was forced to dispose of Sejanus as the opposition to his elevation of rank grew to include many leading Romansand especially the commanders of the legions in Germany.