Why, beyond a certain point, does a higher temperature no longer correspond to a higher rate of enzyme activity?

It is at this point (the optimum temperature) that the bonds holding the enzyme together break. We say that the enzyme (a type of protein) has been 'denatured' and can no longer function properly. At temperatures lower than the optimum, a higher temperature causes a the substrate/enzyme molecules to move faster- increasing rate of reaction as the substrate and enzyme are more likely to collide, resulting in a reaction.