The vowel found alongside a sonant is considered to be:

a) A trace of the Pre-Kartvelian original forms which are evidenced in other Kartvelian languages as well (in Megrelian: birchxili, in Svan, in Laz). Corresponding reduced stems in Georgian must have developed by influence of the non-fixed intensive stress (such a stress is also a product of reconstruction) (Arnold Chikobava, Saxelis pudzis udzvelesi agebuleba kartvelur enebshi [The Archaic Structure of Nowns in the Kartvelian languages], Tbilisi: Mecniereba. 1942)

b) A trace of syllabic sonants that are reconstructed on the level of the Pre-Kartvelian language after the data of the Kartvelian languages  (Tamaz Gamkrelidze, Givi Matchavariani, Sonant'ta sist'ema da ablaut'i kartvelur enebshi [The system of sonants and ablaut in the Kartvelian languages], Tbilisi: Mecniereba. 1965)

c) A vowel developed later as a means for levelling the non-regular (accesive - akcesiuri) consonant clusters (Giorgi Axvlediani, Zogadi ponetikis sapudzvlebi [The Fundamentals of General Phonetics], Tbilisi, 1949, p.235; Tevdore Uturgaidze, Kartuli enis ponemat'uri st'rukt'ura, Tbilisi, 1976, p. 125). Such a vowel is also developed in late loan words both in Georgian dialects and in the Kartvelian languages: Svan and Megrelian. A new-developed vowel always shows that it is  influenced by the phonetic surrounding of the stem  (Sergi Zhghenti, Kartvelur enata shedarebiti ponet'ik'a [Comparative Phonetics of the Kartvelian Languages], Tbilisi, 1960, p.31; A. Ch'inch'arauli, Xevsurulis taviseburebani [Specific Features of Xevsurian], Tbilisi, 1960, p.27).

The corresponding forms in Megrelian, Ch'an (Laz) and Svan languages, on the one hand, and  in the mountain dialects of Georgian, on the other hand,  often show the one and the same step of vocalisation (e. g.: Megrelian: dzhirgil-i, Ch'an: dzhirdzhil-i, Mountain dialects of Georgian: ghirdzili (gum /in mouth/), while in another group of Georgian dialects as well as in Old Georgian so-called reduced stems are found (ghrdzil-i). It is noteworthy that in this concrete case not only a vowel alongside a sonant r but also  this sonant - r ( which later became a reason for development of  a vowel - i) are developed later. (A. Ch'intch'arauli, Kartvelur enata leksikidan[From the Lexicon of Kartvelian languages], in: Et'imologiuri dziebani, 1987, p.51).

More information: From the history of stress and reduction.