More likely it was just a trait in some dogs and those dogs got selected, which is the way most breeds got developed in the past. It might have been very slight at first, but would have strengthened over generations as that trait was repeatedly selected, as Darwin's chesty pigeons got chestier over generations. If you want to call that evolution, OK, but it's not the same thing as saying the dogs developed the trait in response to human attraction to it.
Edited by Faith, : No reason given.