What caused Tiberius downfall?

He got killed, just joking. If you mean the original Augustus, the first Roman emperor, nothing caused his downfall. He was beloved by the people and many of the aristocrats alike. For some, he was the only leader they knew. They experienced and appreciated the improvements and prosperity he gave to Rome. At his death he was deified and subsequent emperors all vowed to rule "according to the principles of Augustus". (Whether they did or not is questionable).