Almost always, this is caused by the last few items/objects/materials that were added to the model. It's extremely common for badly made 3D Warehouse files to crash out Sketchup. It's worth noting that a badly built tree model from 3D Warehouse can contain ten times more polygons than your primary model. Therefore, adding many of these - for example trees to populate a garden, or chairs to populate a restaurant - can instantly complicate the model to the point it becomes unstable. This then manifests in increased RAM and processor usage and places greater demands on the graphics card to render the drawing to the screen.