Validation triggers because of the internal RadTabStrip, which is inside RadRibbonBar. Generally RadRibbonBar does not have CausesValidation and ValidationGroup properties. Once we replace RTS with a simplified and lightweight TabStrip the problem will disappear.
In the meanwhile you can disable validation on the internal RTS in this way:
var tabStrip = (RadTabStrip)RadRibbonBar1.FindControl(RadRibbonBar1.ID +
I hope this helps.
the Telerik team