The evolutionary 'reasoning' seems to be that the caterpillars are going to eat many of those leaves anyway and so the tree makes disposable leaves first and only once they have been eaten does it invest in the leaves full of defensive chemicals to dissuade the caterpillars eating them.
TIL: that there's such a thing as 'disposable leaves'. This explains why everything in spring looks so bright green..
Shocking green by @andrewspink@mastodon.green