In an effort to consolidate Pacifica's hypothesis in regards to this, I did a bit of reading on sound change in word formation. Consequently, I am wondering if this might involve an example of ephenthesis, paticularly of excrescence. You can read a little about ephenthesis on Wikipedia, here:
https://en.wikipedia.org/wiki/Epenthesis
The suffix here is certainly not
-tō , because
-tō is appended to the supine stem of a verb, and is also not known to form causatives. I note that
-iō has formed causative verbs from third declension adjectives, as in
alleviō < ad- +
levis +
-iō, as well as from first and second declension adjectives, as in
ūniō <
ūnus +
-iō. Why, then, would
-itō have been used in the formation of such causatives as we are considering, when
-iō already performed that function for adjectives of the third declension? This leads one to think that either Pax is right in conjecturing that:
Unless these verbs are frequentatives, from non-attested forms like *debilio and *habilio, and humilito happens to have an attested simple form humilio!
...(thus giving
debilito,
habilito, and
humilito the meanings: "I continually debilitate", "I continually enable/habilitate", and "I continually humble/humiliate", respectively, with Pax's hypothetical/unattested
*debilio, *habilio, and
*humilio having taken the meanings "I debilitate", "I enable/habilitate", and "I humble/humiliate", respectively) or, alternatively, that it is possible that we are looking at forms created using
-iō, but with the introduction of the "t" sound after occasions of escrescence (perhaps to assimilate the terminal "-
is" of the third declension)? Perhaps I am overthinking this...but I find it intriguing that I can find no authoritative etymology, or explanation of why
-itō can function to produce an apparent causative from an adjective.