Problem is, in such kind of situations you have seconds to react. If you wait a few more seconds or a minute, the vehicle if its a suicide will reach its target and the result you know.
In my home town, army guys had told all the local villagers that whenever on the main highway army convoy is moving, plz ask your people working in fields beside the main road to just stand up and don't make suspicious movements, especially taking hands into their pockets or taking out mobiles. And that is exactly some of the villagers did with the rationale that who are the army guys to instruct us, as you know we pathan love to do the opposite of what we are asked or requested. So end result, army shot a few of them, mostly cases were of injuries and in some cases deaths occurred.
And you will see lot of such incidents happening in KPK, because we pathans love to defy orders and when we get shot then we do dharnas too.
And in this case, chances are that women was in the back seat of the vehicle, thus it seems she was the first one to get shot from the back meaning the vehicle had passed the check point and it was fired upon from behind.
I on daily basis pass through 2-3 check points and see lot of motorists arguing with security officials and in few cases saw them speed their car even though the army guys asked them to change their route, but army guys showed their coolness and did not order to fire and let them speed away.