I assume you put him in attack posture previously. Were there other factors during the turn that would have changed his stance? Big battle during the turn or something?
Out of curiosity, does the Veteran Activation ignore automatic activation through scripts and roll for each general, regardless of whether they are supposed to be automatically activated?
I think it's the other way around, but I'm not sure. I think the script that enables McDowell runs after the activation roll, so it will override the roll. Although now that I think about it, the activation roll on other difficulty levels must run at the end of turn processing. So I'm really not sure.

For reference, this is the script that works its magic on McDowell.
StartEvent = evt_nam_USA_McDowellActive1861|999|1|NULL|NULL|$Prince_George_MD|NULL
Conditions
MinDate = 1861/07/15
MaxDate = 1861/09/15
EvalEvent = evt_nam_USA_ArmyInManassas;=;0
EvalUnqUnit = Irvin McDowell
Actions
SetLeaderActivity = 1
EndEvent