The sodium bicarbonate reacts when you add it to the hot sugar syrup.
This is due to the heat of the solution breaking down the sodium bicarbonate into carbon dioxide, water and sodium carbonate. The actual chemical reaction is as follows:
2NaHCO3 > Na2CO3 + CO2 + H20
This type of reaction is known as a thermal decomposition reaction which means that the heat of the solution breaks the chemical bonds within the chemical compound.The bubbles from the carbon dioxide in the reaction caused the honeycomb mixture to rise.