As you are now aware eggs are laid on or near host plants so when the eggs hatch the food supply is right there. Since Hairstreak hosts are trees, rather than plants and if the eggs were laid traditionally on the leaves, they would fall to the ground in the fall presenting a food source proximity problem the following spring. This is overcome by depositing the eggs on the buds at the base of the leaf stem so the egg will remain in the tree close to the food source when the time is right. As you can see from this page all butterflies follow the same scenario but not necessarily the same timeline when confronted with a seasonal environment. Even within the same species and the same season the timeline can vary between broods.
Adaptation is the key to survival for any butterfly.