It looks similar to something that happened with my mare's shoulder.
But she was healing up from a drastic injury before, and the original wound had just closed up. The swelling started coming full blown and it looked like the skin was bubbling which then opened, oozed, and went to scabs just like your picture.
Figured that something had gotten into the wound that we had missed and when the skin healed over it, it formed an abscess of sorts. Only, I couldn't tell if the swelling went down with the scabs healing as she was always swollen off and on during those healing days.
Is it possible that she got some slivers of sort? And they just haven't all made it out yet?