They missed somebody at Route 91 'harvest' festival so they sent their assassin to Cali to finish the job? or to get the one person they actually wanted in LV or maybe it's a military team they're after? or the more likely scenario they know some or one of the Route 91 ppl are talking to each other or the Feds and they were just hit and the rest warned.
Too coincidental it's the same group of ppl.