Zompist Bboard Again

Is there a coherent explanation for initial /ps/, /ks/, and presumably an older /ts/? Obviously these come from /p t k/ + s, but it seems to be more complicated than just zero-grading.

The cases where I know the etymlogy actually are zero grades (e.g. /ks/ as zero grade of *ghes- ,g'hes-, ges-, g'es- ,kes- ,k'es-). It seems that some dialects have e.g. /ps-/ from /phth-/ (Cretan). The rest seem to be either loans or unexplained / potential substrate words.