When demand increases for any material,the price will go up.Therefore,when the demand of financial capital increases,you have to pay more to get/buy it.That’s why the interest rate will soar when the demand for financial capital increases.
When expectation of future income increases , demand increases and demand for financial capital (borrowing for spending) will increase…this i turn will lead to a rise in inflation and therefore interest rates will have to increase to curb it.