The start of this verse is a repetition of Matthew 7:25, but in that verse the house built on rock didn't fall. There is also a slight change from the rain beating on the house to the rain beating against it. Matthew, unlike Luke's version does not give a reason for the house to fall, rather the reason is given for why the house built on stone survives.[1] "Great was its fall" may well have been a proverbial term for complete destruction.[2]
This warning of doom and destruction is the final line of Jesus' Sermon on the Mount. To Augustine it is "fear-inspiring"[2]