Environmental causes are a good bet,but there's just too much unknown genetic background on the hamsters(including things that are extremely important but unfortunately you just won't be able to find out).The food most likely was pretty low quality,but that on its own wouldn't be a single factor into death(even though low quality food can be responsible for many health problems developing,including cancer and more especially due to the higher potential for a weakened immune system).
Other than carbon monoxide,there are many potential toxins in household products used by the majority of people.Considering the potential for toxic substances in the air to spread even if used outside of the room a hamster is kept in,that could very well be a contributing factor most people would never expect
