We propose a mechanism based on the rubble-pile hypothesis of the cometary nucleus (Weissman 1986, Nature 320, 242-244) to explain the catastrophic breakup of comet LINEAR (C/1999 S4) observed during July-August 2000. We suggest that a solid nucleus made up of 10-100 m "cometesimals" (Weidenschilling 1997, Icarus 127, 290-306) contains a network of interconnected voids in the intercometesimal regions. The production of super-volatile (i.e., species more volatile than water) gasses into these voids occurs as a result of the thermal wave propagating through the nucleus and associated phase transitions of water ice. The network of voids provides an efficient pathway for rapid propagation of these gasses within the nucleus resulting in gas pressure caused stresses over a wide regime of the nucleus. This provides a mechanism for catastrophic breakups of small cometary nuclei such as comet LINEAR (C/1999 S4) as well as for some observed cometary outbursts including those that occur at large heliocentric distances (e.g., West et al. 1991, Astron. Astrophys. 246, L77-L80). We emphasize the importance of techniques such as radar reflection tomography and radiowave transmission tomography (e.g., Kofman et al. 1998, Adv. Space Res. 21, 1589-1598) aboard cometary missions to determine the three-dimensional structure of the nucleus, in particular the extent of large-scale voids.