Well, they was just after the 2 storm shadow, 1 decoy attack so they might have been a bit trigger happy. Also given that Russia and Ukraine have the same air frames and electronics they have the same IFF units so some mode of IFF will respond the same way. They may have used the wrong mode and gotten a return, which the Russian aircraft had been told to disable. Don't know if this is possibe, but I know that NATO aircraft during exercises would turn off mode 3, but use mode 4. A target the squawked mode 4, but not mode 3 was considered to be friendly. Aircraft that squawked mode 3 only (i.e. commercial aircraft) were evaluated based on their actions. Now IFF is supposed to be coded on a daily or even 4 times a day basis, based on code books sent to the various users. My personal bet it that the ADA units on the ground and the flying units didn't get the same code books, so IFF request goes out, aircraft doesn't squawk the right response, it's hostile. Or the Russians are just idiots, could go either way.