If neither parents had no tail or a partial tail, it is NOT an Araucana trait, but instead a sport, or fluke. Rumpless-ness can indeed pop up in any breed, even Feathersite had a page on that.
The rumplessness from Araucanas is dominant, though. You must have a parent with no tail, or two parents with partial tails in order to have it pass on to the young. Same goes for traits like tufts - You cannot have two clean faced birds throw a tufted chick, although there are indeed internal tufts, those are not common.
Very pretty EE though! I love rumpless birds, which often is a rare thing for people to like. I find it graceful and classy looking on the right bird, and it helps a lot with avoidance of predators, as they go for the tail first.