Your tarantula has just gotten irritated a lot and kicked all of the hairs off of its abdomen. Don't worry, they will all come back the next time it moults. Plus you now have a good way to tell when its getting ready to moult. When it starts refusing food and the bald patch turns black, he or she will be getting ready to moult.
By the way, I don't think that is a B. emilia. They have a black chevron on an orange/rust/red carapace. That T doesn't.
Can you get a clearer picture?
Thank you for rescuing that poor baby and giving it a good home!!!!!!!!!!!!!!