In multiple combats where there is a walker (or several walkers) on one or both sides, the infantry units have to check Morale if they lose as normal. In the event that any of the infantry falls back, the walkers remain in combat. As normal, winning units (including walkers) can only sweeping advance or consolidate if all of the units fighting them fall back.
Multiple Combats
Source: 3rd-A-Palooza (3.5) Rulebook v1.4, p. 45