In another thread, the idea was advanced that a unit had to move to be hit by an air interdiction strike. After some testing, I can confirm this is not the case. In a simple scenario with an infantry unit on one side and an air unit on the other side, the infantry unit was subjected to an interdiction strike without having engaged in any activity. This explains why the SAM unit mentioned above was hit by interdiction strikes as well. Essentially what happens that is the computer looks for "targets of opportunity"; any such targets will (or, perhaps, only may) be subject to an air interdiction strike. This sequence can be seen in the TOAW log file.
AIRCombat:Target of opportunity: Air Defenders 1st Formation, ADA Battery.
AIRCombat:Interdiction attack (bombardment): Air Defenders 1st Formation, ADA Battery.
Combat : Smite: Air Defenders 1st Formation, ADA Battery, (bombardment), attrition%= 9.
Combat : Air Unit weapons firing on Air Defenders Rifle Squad.
Combat : Potentially effective hit on Air Defenders Rifle Squad by Air Unit weapon.
Combat : Rifle Squad destroyed. (specificAttrit=10)
... and so on. I want to do more testing to see if interdiction attacks directed by the computer, and bombardment strikes directed by a human player, result in differing effects against a target in given conditions.