Matthew 7:24 “Therefore everyone who hears these words of mine and puts them into practice is like a wise man who built his house on the rock. 25The rain came down, the streams rose, and the winds blew and beat against that house; yet it did not fall, because it had its foundation on the rock. 26But everyone who hears these words of mine and does not put them into practice is like a foolish man who built his house on sand. 27The rain came down, the streams rose, and the winds blew and beat against that house, and it fell with a great crash.”
It is a special effect of imagery.
From the cinematography point of view, the story and the chapter ends with a big BANG!
Jamieson-Fausset-Brown Bible Commentary
and it fell: and great was the fall of it—terrible the ruin! How lively must this imagery have been to an audience accustomed to the fierceness of an Eastern tempest, and the suddenness and completeness with which it sweeps everything unsteady before it!
The Lord did use the phrase, indicates that it was not a causal expression.
Right. Further, it was an exceptional ending.
We see, from the present example, that it is not necessary for all sermons to end in a consolatory strain.
In case, people miss the importance of this. Vincent's Word Studies explicitly point to Bengel
Great was the fall of it
The conclusion of the Sermon on the Mount. "Thus," remarks Bengel, "it is not necessary for every sermon to end with consolation."
The crash serves as a contrast to the stability of the house built on the solid foundation.
May be he was prophesying the ready-to-happen fall of Jerusalem.
Not in Matthew 7. Jesus did not say that Jerusalem was built on sand, physically or figuratively. In fact, he plainly prophesies the fall of Jerusalem in Matthew 24:1-2.