why does /odom frame drift while running gmapping on rviz?

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I am trying to create a map using a 4 wheeled ground rover. I am using robot_pose_ekf for the pose estimation (wheel encoder+IMU as inputs to the filter), RPLIDAR A2 for the laser scan, gmapping for the integration of the two sets of data and generation of the map. While generating the map the /odom frame tends to keep drifting. Wanted to know what are the possible reasons for this. I have attached the tf tree for reference.