Sometimes when we open the dynamics for agent, the agents directly broken and go up very fast.
Like below picture shown:
That is means this agent been pushed by the default terrain. (See more on Terrain)
If there is no geometry was marked terrain, our system will create a default terrain plane. At this time, if your agent is underneath the ground, it will be pushed up out to ground, very fast and intensive!!
If your agent must be push underground, we have to create a very small plane in scene like below picture and mark it terrain.
At this time the default terrain will not be created and then your simulation will be correct.