As rigor mortis advances, the muscles ratchet tighter and tighter, as they are unable to relax, and her arms could have been pulled up into that position as that process continued. As far as a reflex in response to being hit- I don't think so. If she had put her arms up in a defensive posture, they wouldn't have stayed that way. At the first moment of death, the entire body goes limp, as all muscles enter the stage called "primary flaccidity".
There are cases where severe head injuries cause the arms to curl up like that after death. But in this case, I think her arms may have been placed at her sides or across her stomach when she was wrapped in the blanket and rigor mortis pulled them up.
Or, since evidence suggests she was on her stomach when strangled (the anterior urine stains and the knot at the back of the neck), her arms may have been in that position (bent at the elbow against her chest underneath her) and remained that way. However, this is less likely because of the livor mortis pattern, which tells us that she was placed on her back within the first 15-20 minutes of death, probably before rigor in her arms would have gotten to that stage.