Unwanted movement

We are using UR10e for a research work related to mobile manipulation for more than a year. We are using ROS and Moveit to control the manipulator. Yesterday, while doing some manipulation task, we have noticed some unwanted movement in the UR10e. For example, in one instance, we wanted to move the elbow joint by 40 degree and the joint rotated 40 degree. But, then suddenly it dropped few more degrees. I think this is not coming form the software issues, because we tried the same code multiple times and this sudden drop happened only once.
Can anyone give some idea what is causing this sudden drop and how to stop this sudden drop? Thanks in advance.