Element not showing properly

Hi, we are using OBD (VERSION 10.08.00.44) for our project. 

All this while the model is ok but suddenly the 3D Model illustration/smooth/transparents view are not showing properly,

Most of the 3D elements become lines (Slabs and column showing only lines in Illustration view, but in wireframe is okay. 

Anyone knows whats going on? I can fix using TFRebuild key-in. but need to know why its happening, or else will keeps on happening in the future.

thanks