AIPath random rotate when moving

Hi There.
This issue makes me very crazy, please help me.
I use GridGraph, AIPath, Seek, RVOController on 20 agents and a RVOSimulator in the scene.
The agents random walk on a XZ-plane. It is strange that they always random rotate quickly like shaking or jitter when moving. And also their rotation seems not smooth.
Could you pls tell me why this happened and how can I fix this.
Many Thanks.