Generally, in SLA (second language acquisition), non-native accent (along with any other proficiency issues) would be attributed to:
- L1 interference or transfer
- Imperfect learning
The assumption behind L1 interference is that the first language is never completely deactivated. However, no detailed credible model exists for how this would work. The effects are well described but the causes are still hotly debated. Issues with pronunciation are often attributed to some hypothetical critical period in acquiring phonology but in some cases other factors like hearing loss in older learners can also come up.
Of course, the question is also how to define a 'native' accent. There are many English accents (e.g. Indian) that sound non-native to people in the US or the UK but belong to native speakers of English. But in many of these cases, this new 'native' accent is the result of past L1 interference in the development of the language.