The idea is that carbon fuel dependent industries, which is a lot of industries and infrastructure, have issued vast amounts of debt to fund investment and medium to long term expansion and operations. If the transition to e.g. electric vehicles and solar power generation happens rapidly many of those investments will become massive liabilities and the debt will go bad. This would also lead to a crash in oil prices, so only the cheapest producers would be viable, leaving the rest sitting there on huge oil production infrastructure rusting away, with even more bad debt it will never be able to repay.

I'm not sure how likely/unlikely this is, but that's the theory.

https://www.theguardian.com/environment/2014/mar/06/could-fo...