﴿ ذَٰلِكَ بِأَنَّهُمْ شَآقُّوا۟ ٱللَّهَ وَرَسُولَهُۥ ۖ وَمَن يُشَآقِّ ٱللَّهَ فَإِنَّ ٱللَّهَ شَدِيدُ ٱلْعِقَابِ ﴾

“this, because they cut themselves off from God and His Apostle: and as for him who cuts himself off from God and His Apostle - verily, God is severe in retribution!”

That is because they defied they opposed God and His Messenger; and whoever defies God indeed God is severe in retribution against him.