Tech markets often feature high market concentration and frequent acquisitions. Understanding the motives underlying these acquisition strategies is therefore a first-order question for regulators and practitioners. I formulate a dynamic discrete choice model involving multiple agents to analyze acquisition records of the "big five" -- Google, Amazon, Microsoft, Meta, and Apple. I characterize the firms' acquisition decisions observed in the data as a Markov perfect equilibrium that are driven by their internal motives and defensive motives. I find that the defensive motives can explain a major share of their acquisition behaviors, sometimes overshadowing their internal motives of economies of scale. I show that a company might opt to acquire not necessarily because it enhances its core strengths, but to defend against its significant rivals (other members constituting 'tech giants') from obtaining targets that put itself at a competitive disadvantage.