Mrs. Watson, or Momma, was raised in Birmingham, Alabama, and, because of this, she naturally speaks with a Southern accent. When she got married fifteen years previously, she moved to Flint, Michigan with her husband, and in the intervening time, picked up the nuances of the way people speak in the North. Having lived in Flint for so long, Momma normally speaks like everyone else in the city, but when she is upset, she reverts back to the way she learned how to speak as a child, which is "Southern style."
In Chapter 5, Momma catches Byron playing with matches, something she has warned him a million times not to do. Momma is especially fearful of fire, because when she was a little girl, her house caught on fire, and although fortunately no one was hurt, she and her brothers had to wear clothes that smelled like smoke for almost two years. Momma had just reprimanded Byron about playing with matches a week earlier, and had sworn to God that if she caught him doing it again, she would burn him. Byron does not get the message, however, and Momma is truly irate when she catches him burning toilet paper parachutes in the bathroom even after her threat. Kenny says,
"We all could tell Momma was super-mad 'cause she started talking in that real Southern-style accent" (Chapter 5).
Momma had reverted back to her natural way of speaking once before in the narrative, when Byron got his lips frozen to the mirror of the family car. That time, Kenny explained,
"Momma started talking Southern-style when she got worried. Instead of saying 'here' she said 'he-uh' and instead of saying 'you all' she said 'y'all'" (Chapter 1).