The Shaking - 99:5
Because your Lord has revealed to it.
In the context of verses 1, 2...5; The state of the earth at the end of the world:
The Cave/18:47
On the Day when We sweep away the mountains and leave the earth bare, We will gather them without leaving a single one.
وَيَوْمَ نُسَيِّرُ الْجِبَالَ وَتَرَى الْاَرْضَ بَارِزَةًۙ وَحَشَرْنَاهُمْ فَلَمْ نُغَادِرْ مِنْهُمْ اَحَداًۚ
Ta-Ha/20:105
"They ask you about the mountains. Say, "My Lord will crumble them and scatter them."
وَيَسْـَٔلُونَكَ عَنِ الْجِبَالِ فَقُلْ يَنْسِفُهَا رَبّ۪ي نَسْفاًۙ
Ta-Ha/20:107
You will not see therein the slightest curve or bump.
لَا تَرٰى ف۪يهَا عِوَجاً وَلَٓا اَمْتاً
The Sure Truth/69:14
When the earth and the mountains are lifted up and crushed together and scattered,
وَحُمِلَتِ الْاَرْضُ وَالْجِبَالُ فَدُكَّتَا دَكَّةً وَاحِدَةً
The Ways of Ascent/70:9
The mountains will be like discarded wool.
وَتَكُونُ الْجِبَالُ كَالْعِهْنِۙ
The One Covering Himself/73:14
The day will come when the earth and the mountains will tremble and the mountains will be like scattered sand.
يَوْمَ تَرْجُفُ الْاَرْضُ وَالْجِبَالُ وَكَانَتِ الْجِبَالُ كَث۪يباً مَه۪يلاً
The Calamity/101:5
And the mountains will be like discarded wool.
وَتَكُونُ الْجِبَالُ كَالْعِهْنِ الْمَنْفُوشِۜ