It's a lucky commander who can rely on his troops to perform fearlessly all of the time. In the chaos and confusion of battle troops can easily become demoralised, disorientated or outright terrified by the violence unleashed against them. To represent this element of the unknown your units of troops have to check to see if their morale remains good under certain circumstances. As you will have already gathered Morale tests can occur at a number of different times, so a unit in particularly dire straits may be forced to take several Morale checks in a single turn.
Morale
Source: Core Rulebook 3E, p. 70