It's possible that she has very bad associations with the birthday song. If my friends "forgot" that I hated a particular song to the point that I run out of the room to get away from it, I'd probably hang up too.
Unless your family was bludgeoned to death while the madman was singing this song over and over, and other people knew the story is the only acceptable reason to hang up on something like this for "bad associations"
I am so over people expecting everyone else to know what their "bad associations" are and expect the whole world to change for them. There is a time you need to take a deep breath and get over things. Especially when someone is going out of their way for you.