Your tree was still short enough that the deer may have had a snack. It could also be storm damage, though. Weeping willows have extremely weak wood. It's not uncommon for this to happen to them even at a young age. The good news is that you will easily be able to root the branches. Willows contain a hormone that promotes rooting so all of the branches that you collected will probably root. If deer are responsible for this episode, you will have to surround your new trees with a temporary fence to keep the deer away until the tree is tall enough to be out of their reach.