Occasionally a sweeping advance or similar circumstance will lead to a situation where attackers engage some models in a enemy unit but not all of them. Models in base contact can not move away in their turn, but any unengaged models may move around as normal providing they maintain unit coherency. At the start of the next assault phase unengaged models may move in to help out their comrades as long as the unit did not remain stationary to fire with heavy or rapid fire weapons.
Partial Contact
Source: Core Rulebook 3E, p. 68
When a sweeping advance engages only some of an enemy unit's models, the unengaged models may move freely but can rejoin the fight next assault phase.