New oak leaves. Many plants put out leaves with red tones when small, but then the chlorophyl develops and covers up the red tones as the leaves develop.

Gotcha, thanks! I was hoping that I might be seeing a new bright red plant or something ;-)

Many tree species set dormant buds at their base, for just this reason. Young seedlings can be browsed by animals, stepped on by large things (clumsy people), or burned off by fire.

These dormant (adventitous, epicormic, other terms that better arborists might mention) buds are a survival mechanism, and are likely to occur on any of the trees that you had cut down because there is still a store of energy in those trees' root systems.

They want to live...

That makes me sad :-( There was one tree that they cut down, and the limbs laying in my yard were still growing new leaves! It didn't know it was dead yet...

I have to not think about it, or I'd never be able to do anything! But they had to be cut back, they were close to my house and leaning, if not completely rotted at the base. They cut back several oaks and pines, and you could watch carpenter ants just pour out from the inside!