Tough sunflower

Back in April, a sunflower started growing in the front of the house. I hadn't planted any so it has to be a seed from a plant that grew last year. I did plant a bunch of seeds at the end of May, but not a single one came up. So, the one plant was growing really well, but then got knocked over somehow. It had gotten to almost 4 feet high and had a pretty thick stalk, just like the one's last year. I have no idea how it got broken, but I just left it there and the almost completely severed top half just kept growing. And then a few weeks later some animal chewed it up. I think it might have been the rabbit I see around every once in a while. A couple of days later that part of the plant was just gone. Again I did nothing. I've never even watered anything in the front. Now, a few weeks later, its growing fine again, but has split into four stalks instead of just the single one like it usually is. I'm just amazed and delighted that this plant just keeps on growing.