Ancient greek verbs ending in -sso: a case of paradigm reorganization?

In classical Greek, a small group of verbs yields a -sso ending (Att. -tto) in the present tense, instead of the expected -hw, when it matches the present suffix -ye-/-yo-. The origin of this phonetic irregularity is clarified by means of a paradigmatic analogy with the forms of the inflexion in -s-. The question is why only few verbs have undergone an analogical levelling from -hw to -sso, while the other ones regularly show a present form in -hw, with no analogical restructuring. A solution could be represented by the formal similarity that characterizes these irregular verbal forms: 1) from a prosodic point of view, they represent disyllabic paroxytone forms, and 2) they are characterized by the presence of a radical vowel /a/ and by a consonantal onset. In this perspective, this small group seems to constitute a natural radial category, arranged around a prototypical centre: when the phonological change results in different outcomes, depending on the contextual elements, a paradigmatic irregularity arises in the system, increasing the lexical memory charge (storage). On the other hand, the establishment of a natural category, triggered by the presence of one or more common features, straddling lexicon and grammar, facilitates the mnemonic expenditure and makes a paradigmatic waste more consistent