Merger of Early Modern English 'ir' with 'ur' and 'er'+'ear'

Before /r/, /ɪ/ merged with either /ʊ/ or /ɛ/, depending on context. After labials (plus clusters of labials and /l/) and alveolar stops (like in bird and dirt), the result was /ʊ/ (shown, among other spellings, by ‘durt’, which was an old spelling variant of ‘dirt’), which then became /ʌ/ in most dialects, while /ɛ/ is the result elsewhere. They both eventually merged before /r/ into a single central vowel in most modern English dialects. While /ʊ/ can be expected after labials, it being the result after alveolar stops too is unexpected. What are the articulatory and perception bases for the merger of /ɪ/ into either /ʊ/ or /ɛ/ before /r/?