Thales S. Teixeira distills years of research on how companies are disrupted in his HBR article Disruption Starts with Unhappy Customers, Not Technology:

For eight years I’ve visited leading companies in more than 20 industries around the world that claimed to be in the process of being disrupted. Each time, I’d ask the executives of these incumbent companies the same question: “What is disrupting your business?” No matter who I talked to, I would always get one of two answers: “Technology X is disrupting our business” or “Startup Y is disrupting our business.” But my latest research and analysis reveals flaws in that thinking. It is customers who are driving the disruption.

He goes on to explain why disruption is a customer-driven phenomenon, and how incumbents should respond.