I was on my XBox playing Ghosts on Local this morning. I checked for the Invasion DLC but it wasn't there. Then as I was playing in Local my Xbox Live suddenly dropped out and said I had to update.
So I did the update, went back into Ghosts and then the Invasion DLC had appeared in the shop, foc for me as I have a season pass.
I assume rightly or wrongly the update was caused by the DLC becoming available.