Erhua

Erhua (Tsushiman: _) is a phenomenon in which "r-coloring" or the addition of either /ɹ/ or /ɚ/ to words. In Mandarin, this changes the meaning of the word, but in Tsushiman, this is a phonological process with an even greater extent. Specifically, from Old Tsushiman to Middle Tsushiman, nucleus /i/ became /ɚ/, while coda /n/ and /ŋ/ became /ɹ/. If the sequence /in/ or /iŋ/ occurred, it would become /ɚ:/.

In addition, intrusive R - the addition of /ɹ/ between two of the same vowel - developed, despite not being a feature of erhua in Chinese.

Cause
The cause for "normal" erhua is known - due to the raids of the Wokou. Many Northern Chinese people were kidnapped to forcibly work in Tsushima (and later Jeju), who brought along erhua with them. In contrast, Southern Chinese dialects have no erhua at all; the Wokou would only raid these parts by the 1400s, and by that time erhua had solidified itself in Middle Tsushiman.

On the other hand, intrusive R erhua is much harder to source, as in most other languages, two adjacent vowels would be separated by a glottal stop.