I believe it is Robinia Hispidia! And it is mainly spread by suckers (roots that go out and form another plant), but also spreads by the sticky seed pods. It is hard to say exactly how the plant got into your yard, but a seed pod probably came on the fur of an animal. It starts blooming the second year, so it has been around for a while. It is not dangerous, in fact it is planted along road banks to prevent erosion and is sometimes planted in gardens due to its beauty when blooming. The USDA does not recommended it for residential gardens as it can spread aggressively unless you are careful about removing the sucker plants