It was part of the prophesy. I believe it came from the fact the scripture says Israel will take 7 months to bring in all the armament left on the battlefield in the valley after the war. I believe it is thought that this is some kind of indication of how much time is left before the church leaves the earth .. more or less a year's period of time.
If my memory of those Scriptural prophecies is accurate, the Bible says that Israel will live off the remains of the battle for 7 years.
Some have taken the fact that some of the armament on Russian tanks is a wood based product. And that therefore, it will result in Israel burning the pieces of that wood product for 7 years.
We shall see.
Hard for me to speculate with so much specificity at this point.