"Too big to fail" is a reference to the 2007 US banking crisis when the housing bubble popped and too many people defaulted on their subprime mortgages. They couldn't repay their loans to the bank. This had a cascading effect because the banks didn't have enough cash reserves due to overleveraged debt and fractional reserve banking policies. When depositors panicked and tried to withdraw cash, there was no money. The entire shadow banking system was built on rehypothecating debt so everything collapsed and the US went into a major financial depression. The debt system is like a huge jenga tower and the fractional reserve system backing all the liquidity was the bottom block pulled out by the housing bubble pop. The banks went bankrupt but were deemed to be "too big to fail" and were subsequently bailed out by taxpayer money.

This has all happened before (very recently) and no lessons were learned by those at the top.