A unit which is assaulted by the enemy whilst falling back must check to regroup immediately. No modifiers apply to the check in this case, and even units which would not normally be allowed to regroup are allowed to make the check - the situation is sufficiently life or death threatening for the usual regrouping restrictions to be ignored. If successful, the unit is regrouped and will fight the close combat normally. If it fails to regroup, the unit is scattered and counts as destroyed. Note that this rule applies if a unit consolidates into enemies falling back too.
Assaulted While Falling Back
Source: 3rd-A-Palooza (3.5) Rulebook v1.4, p. 28