It could be a couple of things. First, if you saved your file but didn't update your scene, then came back later and did, things would jump. Second, a change we added in 3.0 M1 (release 3.0.4810 / 4811) is that when you edit a model in LayOut, the scene gets marked modified AND turns OFF "Preserve Scale". If we didn't do this, you'd edit your model, get it set up, exit, then the model would jump (because of the scale). Lastly, there are some instances where http://sketchup.google.com/3dwh/buildingmaker.html buildings would make a scene jump, and it has to do with reconciling 2D and 3D cameras. But that doesn't sound like your case. Do either of the first two choices fit?
b