Why did the duke of Austria release Richard?

The Duke of Austria released Richard the Lionheart largely due to the political pressure and changing circumstances surrounding Richard's captivity. As Richard was a significant figure in the Crusades and his release was seen as beneficial for maintaining alliances, the duke recognized that holding him could lead to further conflict. Additionally, Richard's mother, Eleanor of Aquitaine, and various English nobles were actively negotiating for his freedom, which likely influenced the duke's decision. Ultimately, the combination of external pressures and the potential for political gain led to Richard's release.