That is because they opposed Allah and His Messenger. And whoever opposes Allah and His Messenger - indeed, Allah is severe in penalty.
The Saheeh International Quran Translation
ذَٰلِكَ بِأَنَّهُمْ شَآقُّوا۟ ٱللَّهَ وَرَسُولَهُۥ وَمَن يُشَاقِقِ ٱللَّهَ وَرَسُولَهُۥ فَإِنَّ ٱللَّهَ شَدِيدُ ٱلْعِقَابِThat is because they opposed Allah and His Messenger. And whoever opposes Allah and His Messenger - indeed, Allah is severe in penalty.