Bless the birds, they plant seeds where ever and when ever. It could also be a sport of a plant you intentionally planted. Gardening is a constant surprise, isn't it?
The same sort of thing happen here. It's either birds or squirrels that are responsible but I realty don't mind. The latest is a briar rose in the back garden that I know I didn't plant.
That is the same question some of our neighbors ask every year when Fig trees start growing in their lawns. Our Fig tree becomes the world's largest bird feeder in late Summer and the next Spring we see many baby Fig trees in yards along our street and others in the neighborhood that the birds planted for them. Some get to stay and grow but most get chopped down by lawn mowers. I have several plants that have been 'moved' from our front garden to the back garden and from a neighbors garden to ours. Two of those are.... Capsicum annuum var. glabriusculum (Texas Bird Pepper, Bird Pepper, Pequin) Ampelopsis arborea (Peppervine)
I think it is a penstemmon, as it looks like the taller lavender (light purple?) one next to it and I know it is a penstemmon. I'll take a better picture of it tomorrow. I like it, so this fall I'll transplant it so it has a little more room to grow as it looks like a 'keeper'. Most of my 'voluntary' plants are fir tree seedling, elm trees, box elder trees. I've almost eradicated the jonny jump-ups and grape hyacinths.
Oh! Send the johnny jump ups and grape hyacinths to me!! Funny how one person's treasure is another person's junk!