eNodeb receives CQI report from UE. It may be at eNodeB that UE is not sending proper CQI according to current channel condition and eNodeB assumes that UE has good channel condition and based on that eNodeB using MCS. In such a case, when eNodeB and UE both are not in sync w.r.t to channel condition. It may occur in system.
Please verify air interface log at UE as well as eNodeB. It may help out to you.