Robot slides on plane with no actuation. Unexpected continuous motion

Hello,

I’m simulating a 4-leg robot (spherical foot at each tip) with a CMG in the center in Simscape Multibody. I run the model without any external forces or actuation and with small damping/stiffness in revolute joints. Each spherical foot uses a Spatial Contact Force block with static friction = 0.6 and dynamic = 0.5. Gravity is normal (plane is horizontal, parallel to XY plane).

Problem: when I run the simulation from rest (no actuation), the whole robot begins to glide on the plane and accelerates — continuous sliding that should not occur. No external actuators are enabled and gravity should not produce horizontal motion.

Where to look and what to test. Any known causes or fixes?

Thanks!