Well, looks like you enabled property "Smooth Bends".
If this is indeed the case, you can select your edges and turn that feature off again.
If new edges start with "Smooth Bends" enabled, you probably changed the default edge style. To use another style as default style, find the style template you would like to use in yEd's palette and either double-click the template or right-click the template to open its context menu and choose "Use as Default".
