This is because sugar molecules don't crystalize with the water.
In chemistry, fractional crystallization is a method of refining substances based on differences in solubility. If two or more substances are dissolved in a liquid, they will crystallize out of the solution at differ ent rates.
Crystallization is usually caused by changes in temperature – most often, cooling down.
For example, if sugar is dissolved in water and the solution is partially frozen, the liquid portion will taste sweeter than the frozen portion because the sugar will be more soluble, or more likely to dissolve, in the liquid.
So when the fruit punch freezes, all the water molecules form ice. When it's half frozen, you are tasting more of the flavor than the water.
To learn more, see Separating Mixtures: Techniques and Applications from Lexingtonma.org.