What explanations are there for exhailing and movement hours for hours after death?

my aunt died recently , during and after death was pronounced by medical professionals, to be exact she continued to exhale for five hrs. my other aunt who was with her felt so distressed and unable to leave her, however before the final exhale my deceased aunt raised her shoulders, as if she were trying to sit up, it was at that point the exhailing stopped. The medical team apparently stated they hadn't experienced this there. Well , my grandma shared this with me and i reured her i would try to find some info on this phenomena. perhaps someone could shed some light on this for me?