According to orthodox economic theory, large US corporations must be buying back stock with earned profits and new debt[a] because they don't have better uses for that money. Rational executives and company boards looking out mainly for shareholders must have concluded that buying back shares at current prices is a better use of money than investing, you know, in the business.
Maybe.
In reality, executives and directors could very well be authorizing stock buy-backs to keep share prices up so their stock options remain in-the-money for as long as possible. If that's the case, the buybacks are meant more for the benefit of executives and directors than for the benefit of the business or its shareholders.
[a] Corporate debt to GDP is now at an all-time high: https://ei.marketwatch.com/Multimedia/2018/11/29/Photos/NS/M...