Hard to tell everything just looking at the pics but it looks like your board was quarter sawn, yes? That is a very beefy looking riser. I suspect the limb was just doing too much work, and the wood just couldn't handle it. Not all boards will make a bow. Some are too dry. Some just don't have the cellular structure required.
Did you ever pull it past it's intended draw weight during tillering?
I'd have gone longer NTN with a red oak board bow myself. Maybe 64-66". Make a longer one next time. As you get better, you'll make shorter ones if you'd like. Maybe start your limb taper closer to the mid limb area.
Everyone breaks bows, but you learn from each one and build better bows as you go.