The answer can be found in a concept that is sometimes labelled “systemic relevance” and sometimes “too big to fail”. Now, what does it mean when a bank is systemically relevant or “too big to fail”? In essence, it means that the bank in question is very big or very interconnected.
It is woven so deeply into the fabric of the financial system that its failure might bring down the system. The most prominent example is the fall of Lehman Brothers in September 2008. This event marked the beginning of the global financial crisis we are still in.