Is it possible that there are some tree seeds in the leaves you are using to mulch?
I have large beds and a lot of them and sometimes a tree seedling will get hidden around other tall plants and I won't see it until it is too big to pull. If I try to dig it out I disturb roots of nearby plants. I have started cutting the seedling off at ground level and immediately spraying the cut end with poison ivy/brush killer. I can usually make the spray a narrow stream and spray just the cut end of the seedling. This seems to work for me. It kills the seedling and doesn't harm the plants around it.
