In the case of flooding, it is understandable how an electrical fire can occur and spread. Unfortunately, there are now far too many cases of fires that clearly originated in the cell itself.
I speculate that in our quest for ever more energy density, which by the way happened 20+ years ago, the chance of a cell internally shorting is increased. Dendrites grow and directly short out a cell. Components that are not perfectly manufactured can eventually touch and short out a cell, overheating remains a problem, and so on.