This is due to the components of the free energy equation (ΔG = ΔH -TΔS) that add up to be negative or positive. While an exothermic reaction (-ΔH) may suggest spontaneous reaction, a high temperature (T) or large entropy (ΔS) may negate the negative enthalpy and result in a positive ΔG value.

​https://jackwestin.com/resources/mcat-content/energy-changes-in-chemical-reactions/spontaneous-reactions-and-dg