I had a good geomorphology teacher. He was so good that I bought his book, published on Prentince Hall at 2008. He is an aged an experimented geomorphologist, so I totaly trust there is not an agreement about the physical origin of tafonies (or at least there wasn't an agreement at 2008 when his book was published).
"Its origin [tafonis] is controversial (Twidale and Vidal Romaní, 2005). Some authors differentiate between wall tafonis and basal tafonis, linked to the processes of basal undermining (Smith, 1978, Bradley et al., 1978). It is agreed that the dominant processes are those related to the movement of water with salts close to the surface, which correspond to haloclastism and wetting and drying. Thermoclastism, cryoclasty, dissolution of carbonated cements and wind erosion are also mentioned (Evans, 1969-70, Mustoe, 1983, Young, 1987, Robinson and Williams, 1992). In addition to these subaerial processes, some taphonis can be generated by weathering under the soil surface and subsequent exhumation by erosion (Dragovich, 1969)."
Source: Geomorfología, Prentince Hall (2008), page 152.
Translated from spanish by Google.
-Gutiérrez Elorza, M. (2008) "Geomorfología". Ed Prentince Hall.
-Twidale, C. R.; Bourne, J.A., y Vidal Romaní, J.R. (2005): "Beach etching and shore platforms" Geomorphology 67, 47-61.
-Smith (1978) "The origin and geomorphic implications of cliff foot recesses and tafoni on limestone hamadas in the northwest Sahara" Zeitscrift für Geomorphologie, 22, 21-43.
-Bradley, W.C.; hutton, J.T., y Twidale, C.R.(1978). "Role of salts in development of granitic tafoni, South Australia". Journal of Geology 86, 647-654.
-Evans, I.S. (1969-70) "Salt crystallization and rock weathering: a review" Revue de Géomorphologié Dynamique, 19,153-177.
-Mustoe, G.E.(1983)"Cavernous weathering in the Capitol Reef Desert, Utah" Earth Surface Processes and Landforms 8, 517-526
-Young, R.M. (1987) "Salt as an agent in the development of cavernous weathering" Geology 15, 962-966.
-Robinson, D.A. Williams, R.B.G.(1992) "Sandstone weathering in the High Atlas, morocco" Zeitscrift für Geomorphologie 4,413-429.
-Dragovich, D. (1969): "The origin of cavernous surfaces (tafoni) in granitic rocks of Southern Australia". Zeitscrift für Geomorphologie 13, 163-181.