Sucker!
I was out in the back yard checking on my upcoming tangerine crop today, and I noticed what looked like a big, weird weed behind one of the tangerine trees. Following it down, I could see it was growing out of the base of the tree. But it had different leaves and big thorns on it, unlike the rest of the tree. So I looked it up, and it was a sucker. They say to cut them off when they’re small, but it was far too late for this monster. i had to get the bow saw to cut it off. But at least now I know to check the tree periodically for this.