Child developing an "accent", sort of?

We're in the southern US, but not an area with a strong regional accent
(dialect, among some populations, yes, accent in standard American English,
no). Recently, my 4 yr old has started using a dipthong on vowels in some
words, which sounds, to my ears, like a Texas twang. Neither her father nor
I have this accent (I have some strange speech sounds due to my speech
impairment, but she hasn't picked up on those), and neither do her teachers
at school.

It's pretty obviously an affectation, because it comes and goes-it seems to
be something she puts in when talking to a person, but not when she's
playing by herself, putting in voices for toys, and so on. She also
sometimes forgets mid conversation and starts talking in what, a few days
ago, would have been her normal voice.

I don't think it's a problem-but it does seem rather unusual to me.

Anyone else's child do this?