I believe the carbon dioxide is liquid going down the well, then becomes gas and expands as you said - expands is a better description than exploding. I know sand and pellets are used to hold fractures open. Also know water is used. What I don’t see are toxic chemicals the writer said was used and the nearest component to “toxic” would be the carbon dioxide which wouldn’t be toxic to end up in human beings as I believe the writer intended to imply. The point is, none of those “ingredients” used is toxic.
There are some things in the mix intended to kill bacteria, and other substances which inhibit corrosion and adjust the properties of the frac fluid. These are generally used in very small quantities. They don’t get into groundwater unless someone spills them on the ground, and unless the spill is huge, there isn’t enough to be a hazard to anything.
Carbon dioxide is injected into oil fields as a enhanced oil recovery method. It is rarely used to fracture. It is easier to think of it as used to sweep through the reservoir to help lift out hydrocarbons sticking to the rock.
Read more about the process here:
http://neori.org/resources-on-co2-eor/how-co2-eor-works/