Base chance that a battle will aggro in a second monster to fight you together, whether a titan, local mob, or a generic "Angry Resident of ?" as practical for coding. Maybe aggro escalation for more jump ins?

By some determination - such as daily time on a planet or in an area, or universal kills/battle in a day or simply such on specific planet - increase that base chance by steps.

Nullify rage could decrease or prevent this too. Enraged makes more likely or certain.