The Ausbund did not begin as a hymnal. Its core is a group of hymns written by Swiss Brethren who were imprisoned in the castle at Passau in the later 1530s, and the texts read as what they are: accounts of interrogation, of refusal, of executions witnessed and expected. They were circulated among Anabaptist congregations in manuscript, printed together in 1564, and enlarged in 1583 into substantially the book that is still in use.
That origin explains the shape of the collection. It is not arranged by the church year, because the people who wrote it were not running a church year. It is not devotional in the way later Protestant hymnody is devotional. A large part of it is martyrology set to metre, naming individuals, places and dates, and it was sung by congregations who understood themselves to be next. Anabaptists were being executed across the German speaking lands in the decades the book covers, by Catholic and by magisterial Protestant authorities alike, and the hymnal is one of the movement's primary records of that.
What makes it unusual as an object is that it never stopped being used. Continental Anabaptism largely moved on, adopting new hymnals from the eighteenth century, and it is the Amish who carried the Ausbund to Pennsylvania and kept it. The book in an Amish hand today is recognisably the 1583 collection, in the German of that century, printed without music, and sung by people whose everyday speech is a different dialect entirely.