Bourse Direct is a french low cost broker. Low cost doesn’t mean low quality. Their brokerage fees are among the best (0,99€ per transaction, I am not a client!). So its main business is linked to the volatility of the stock market, more volatility means more transactions means more business so better earnings for the company.
Another driver is the increase in the number of clients as more clients bring about more transactions. What has changed recently is that interest rates have increased. Now they are fully benefitting from a second pillar to their net banking income: interest income. As they invest the cash of their clients.
The brokerage business is doing fine but clearly, the big boost in profitability comes from the rise in interest rates. In other words it is like a bank: one can benefit from rising interest rates but it isn’t a bank as it doesn’t give out loans and only has deposits. Only the best side of the business.