I'm not familiar with what's in chlorine tablets, my guess is that it's a combination of sodium hypochlorite and calcium hypochlorite with other compounds probably cyanuric acid.
Seeing as sodium and calcium hypochlorites are oxidizers, there is some sort of redox reaction going on where the hypochlorites are the oxidizers, and the ispropanol is the reducer.
That's my best attempt at explaining what's going on chemically, keep in mind I don't have a degree in chemistry, and it's the middle of the summer.
By the way, the physical explanation given by dna444 is right. Chlorine gas is released, eventually the pressure hits a point where it causes the bottle to burst.