That is for what your hands have put forth [of evil] and (8:51)

﴿ذَٰلِكَ بِمَا قَدَّمَتْ أَيْدِيكُمْ وَأَنَّ اللَّهَ لَيْسَ بِظَلَّامٍ لِّلْعَبِيدِ﴾
[ الأنفال: 51]

8:51 That is for what your hands have put forth [of evil] and because Allah is not ever unjust to His servants."

The cause of that painful punishment at the time when your souls are taken out and the burning punishment in your graves and in the Afterlife is because of the evil actions you performed in the world.
Allah does not wrong people, but passes judgement on them with fairness.
He is not unjust to His slaves.

"This is because of that which your hands had forwarded. And verily, Allah is not unjust to His slaves."

phonetic Transliteration

Thalika bima qaddamat aydeekum waanna Allaha laysa bithallamin lilAAabeedi

Abdullah Yusuf Ali - Translation

"Because of (the deeds) which your (own) hands sent forth; for Allah is never unjust to His servants:

Safi-ur-Rahman al-Mubarakpuri

"This is because of that which your hands forwarded. And verily, Allah is not unjust to His servants."

Page 183 English transliteration