At a pack level we need to consider more than the cells when we look at thermal runaway. However, we need to start with the cell.
Battery Thermal Runaway
What causes a cell to go into thermal runaway?
Then we need to consider the Cell to Cell Thermal Propagation.
Thermal Propagation
We tend to use thermal runaway to describe a single cell and thermal propagation to describe the cell to cell spread.
There are ways to manage / mitigate cell to cell thermal propagation — see Propagation Mitigation for a fuller treatment of the barrier, venting, clustering and fusing strategies used.
Arcing
With High Voltage (HV) and a gas loaded with carbon it is inevitable that we will get electrical arcing during thermal propagation — see also DC Arc Flash Hazard and Arc-Management Methods for how that risk is managed at pack level.