Is a person considered to have caused his grandmother's death, and is he liable for the expiation of accidental killing, if the grandmother tripped over a placed stroller and suffered a hip fracture that later led to her death?

If you placed the stroller in a location not frequently used by the household members, then you are not an aggressor (mu'tadīyah) and are not liable for compensation (ḍamān). However, if you placed it in their pathway, you might be an aggressor.

Assuming you are an aggressor: 1. You are liable for the grandmother's fracture, but not for her death unless the death was due to complications of the fracture (sarāyat al-jarḥ - spread of the injury). 2. If two trustworthy doctors confirm that the death resulted from complications of the fracture, and you were an aggressor by placing the stroller, then you are liable for expiation (kaffārah), and the blood money (diyah) is incumbent upon your 'āqilah (male agnatic relatives) for the grandmother's heirs. 3. If the death was not due to complications of the fracture, then you are not liable for anything concerning her death.