Maybe it was flung into Jool by a moon? Happens all the time for me, especially with Tylo and Laythe. Edit: Disregard; I thought you were orbiting Jool, not Laythe. What orbit were you in over Laythe? What mods do you have?
Move this to Gameplay Questions, please http://forum.kerbalspaceprogram.com/threads/83611-How-do-you-handle-mountainous-terrain-on-Kerbin-elsewhere Edit: Nevermind, it's been moved