Many capitalist competitors have been liquidated, so those capitalists left can earn quick profits. That reignites growth. Interest rates are low so capitalists start to borrow to invest. Now the boom really gets under way. Then profitability starts to drop.
Inflation picks up and with it interest rates.
Capitalists are over-extended. They start to cut back on investment. Production slows and then drops sharply. Workers get laid off, stop buying goods and the crisis of production begins. The first signs of this late stage of the cycle are now to be seen.