NEW YORK ( TheStreet) -- Green Mountain ( GMCR)shares are getting ground down today in a predictable collapse triggered by a threat to its popular Keurig single-cup brewing system as Starbucks ( SBUX) prepares to launch a rival concept.A 16% drop in Green Mountain shares, which recently traded at $52.16, makes the stock the second-biggest decliner on the Nasdaq today. Green Mountain shares are now worth less than half their 52-week high of $115.98. Starbucks, meanwhile, is up 2.4% at $51.56, setting a new 52-week high. Green Mountain has been living on borrowed time with Keurig since the patent on the K-Cup single-brew technology is running out. Starbucks, known for its pricing power, plans to charge more for its "Verismo" system even as it continues to supply coffee for Keurig machines. Think this is a buying opportunity? You may want to reconsider. Here are 6 Reasons Not to Buy Green Mountain on the Dip.