If you get into trouble and the enemy senses vulnerability - a bleeping shield perhaps - or if you push too far into their midst or maybe just get too close, Grunts and Jackals can quickly go into a vicious high-aggression mode involving rapid-fire shooting. You'll probably recognize it when it happens, because all of a sudden you'll be getting torn to shreds by plasma pistols and getting desperate. It's like you've made a nest of hornets angry. Elites get more aggressive too, but you may not notice that so much because they're already a nasty pain! The main moral is: don't forget about this! If you get overconfident and go wading into a bunch of Grunts and Jackals thinking you're going to slaughter the lot without any trouble, you may regret it!