Hypersonic weapons: Why is Russia accusing its own scientists of treason?

Russian President Vladimir Putin frequently boasts that his country is leading the world in developing hypersonic weapons, which travel at more than five times the speed of sound.
But a string of Russian physicists working on the science underlying them have been charged with treason and imprisoned in recent years, in what rights groups see as an overzealous crackdown.
Most of those arrested are elderly, and three are now dead. One was taken from his hospital bed in the late stages of cancer and died soon afterwards.