From memory, people were getting large mortgages they couldn't afford once the teaser rate had expired (usually 1 - 2 years). The reason they were able to get these mortgages is because lenders were doing little or no due diligence.

This did coincide with a period when fuel prices were rising but, I think, this was more a case of 'the last straw breaking the camel's back' rather than a direct causation of financial distress.