Deep Space Kraken
From Kerbal Space Program Wiki
The Deep Space Kraken is a bug in the game that is experienced in deep space. It is an error with the games physics engine, PhysX, which attacks spaceships as their velocity increases. The number of parts in your ship has a large effect on what speed the Kraken Manifests itself, as well as how much it affects your ship. It primarily causes your craft to go off trajectory due to forces that shouldn't be there.