What caused liver fracture?

We fostered an abused, rescue dog in January of 2012 to May 2012 when she died suddenly of a ruptured liver. After spending enormous amounts of time undoing the damage someone else did, loving this dog completely and gaining her trust, we are dealing with the horror of guilt and blame for her death. We left town and arranged for two friends to take care of her and walk her. On the day we came back, it was extremely hot, she went for two long walks each with the different walker, even though she wasn't a fan of long walks. That night when we returned, we witnessed her fall down stairs (which was weird, but didn't think it injured her), she didn't eat much (but we didn't think anything of it because it was so hot) and the next morning when we woke, she was dead in her crate. The vet for the rescue said she had two ruptures in her liver, which could only be from blunt trauma - they believe either she was kicked or hit by a car. This is devastating news to us, as either one of the dog walkers kicked her, or she ran out in front of a car with them, or the trauma from the fall down the stairs, possibly onto a sharp metal object could be the culprit. There has been a wave of dogs dying from squirrel poison here in Chicago, and we thought that might have been it before the vet did the autopsy. What has been most devastating, is that some people in the rescue believe there was foul play involved and we are to blame. I am desperately searching for answers. It feels wrong to blame the dog walkers, when they insist they took good care of her and we have NO reason to suspect they would otherwise. We are about to adopt another dog, and I am searching the internet for whether squirrel poison can rupture a liver. The only fact we have is that she was abused before we got her, and now we are suspect of abusing her, which is the strangest irony. I just would like a second opinion, which is difficult without the autopsy I know, but the trauma for me has subsided enough that I feel brave enough to try and find the answers again. Losing a pet is sad enough, but being blamed for her death is something I don't think we will ever recover from. Thanks for your help.