The center of gravity for the rocket is actually very low. The engines are on the bottom, and the rest is just a light aluminum tube (with other smaller tubing and tanks inside but still.)

Also, after the landing they vent the remaining oxygen, and afterwards people come aboard to weld the rocket legs to the surface so it wouldn't slide off.

Maybe very strong winds would be an issue, or very rough seas immediately after landing but they wouldn't launch in rough weather anyway.