The problems which initiated the crisis at Northern Rock during the summer of 2007 were:

Large customer withdrawals despite the UK regulator and the UK Treasury giving assurances that the bank was solvent

Doubts arising about the viability of the business model which necessitated Bank of England intervention

A general lack of confidence in mortgage backed securities associated in large part with developments in the US sub-prime mortgage market, and doubts emerging about the viability of the Northern Rock business model

A depositor run on the bank, following doubts about the viability of the Northern Rock business model