ذٰلِكَ بِأَنَّهُم شاقُّوا اللَّهَ وَرَسولَهُ ۚ وَمَن يُشاقِقِ اللَّهَ وَرَسولَهُ فَإِنَّ اللَّهَ شَديدُ العِقابِ

This is because they defied God and His Messenger. Whoever defies God and His Messenger—surely, God is severe in retribution.