Share buybacks often occur when a company (not just banks) believes their share price is undervalued in the market.
If the company has available capital, it can buy back shares and it could have a positive impact on tangible book value per share, EPS and dividend yield.
So the capital they used would be subtracted from equity, but the share count would go down as well.
It happens when the stock price is perceived to be low, and they have "unearning" excess capital that they can deploy.