Maybe I am misunderstanding, but I am not sure why the newly selected larger supplier would be loosing money of many of their current customer contracts as the energy may have been bought a year or more ago when prices were lower? Except for defaulters the customers are not loss making and are again.
My guess is that energy companies go under because they can no longer service their debts. They operate on a very fine profit margin based on the belief that all their customers will be paying on time by DD.
If you get a number of customers defaulting, the energy company cannot pay its creditors and if it cannot pay its creditors it cannot buy more energy as no money in the kitty. Also it does not help when the Big Six push up the prices of the energy they are buying making it difficult for a smaller supplier to buy energy anyway.