That is because they opposed Allah and His Messenger. And whoever opposes Allah and His Messenger - indeed, Allah is severe in penalty.
English Transliteration
Thalika biannahum shaqqoo Allaha warasoolahu waman yushaqiqi Allaha warasoolahu fainna Allaha shadeedu alAAiqabi
Tafsir of Verse 13
That, because they had made a breach with God and with His Messenger; and whosoever makes a breach with God and with His Messenger, surely God is terrible in retribution.
This because they contended against Allah and His Messenger: If any contend against Allah and His Messenger, Allah is strict in punishment.