I think the Hoatzin is a good starting point. The young have claws on their wings that allow for climbing. These claws are lost in adulthood, so the first evolutionary step is for a Hoatzin to evolve neoteny, retaining the claws and becoming secondarily flightless. It does this to save on developing wings and flight muscles, and to exploit the niche taken by monkeys.
Larger Hoatzinids become primarily land based. They climb trees as needed but rely on their claws for defence. They eat leaves and fruit.
However climate change results in a shrinkage in forests and so some evolve to live on the new grasslands that are taking their place. Here there are predators like eagles, so extra bulk is an advantage. The obvious food source is the grasses so that is what they eat. Their feathers are reduced to a thin covering (except for tufts that the males sport during the rut) and their claws are mostly used for digging for roots and the annual rut. They swallow stones to help break down grasses in their stomachs.