Here is Prickly Ash:
http://uvalde.tamu.edu/herbarium/final/zahi_br.jpg
I'm reading that Prickly Ash has greenish-yellowish flowers.
Whatever it is I'd think one of two things:
Either a mama one nearby that's dropping seeds, or
A recent re-seeding with a bad batch of seeds that contained weed/bush/tree seeds in it?
F that does not look like what you've got, can you post a picture?