Invertase is an enzyme. Enzymes have very limited ranges of effective temperature. This is due to degradation of the chemical structure at higher temperatures. The enzyme may still work at lower temperature but may take longer. The reaction needs a specific amount of energy to proceed and that is where obtaining the lower optimum temperature comes in.
In addition to the question, why do temperatures lower or higher than the optimum cause decreases in invertase activity, I also need to know what is happening to the enzyme to produce these decreases in activity
The enzyme becomes denatured at high temperatures...that means that the chemical bond holding the structure together break down, so the shape of the enzyme changes and can no longer operate as a 'key' to trigger the reaction.
In terms of lower temps, then there is insufficient energy of activation to push the reaction forward.