Fronting may be triggered by a nearby sound, in which case it is a form of assimilation, or may occur on its own.
In the Attic and Ionic dialects of Ancient Greek, Proto-Greek close back /u uː/ were fronted to /y yː/.
This change occurred in all cases and was not triggered by a nearby front consonant or vowel.
In many dialects of English, the vowel /uː/ is fronted to [u̟ː] or [ʉː], a sound change that is sometimes called GOOSE-fronting.
For example, in the Northern Cities Shift, the raising of /æ/ left room in the low-front area of the vowel space for [ɑ] to expand.