This sentence is saying that bagged Starbucks coffee sold in stores has been very popular the past few years. Dunkin' Donuts made a deal with the company Procter & Gamble so their coffee can be sold in stores too, presumably to make money off of this trend.

Robin, I agree with you that the sentence is confusing. I think it should read: "... where bagged coffee from Starbucks has been a growth machine in recent years" . Obviously, the word "coffee" was missing, since it is the coffee which is bagged, and what has been such a growth machine for Starbucks