“The defining characteristic of a tightly coupled process is that once it starts, it’s difficult or impossible to stop: A domino toppling display is not especially complex but it is tightly coupled so is a loaf of bread rising in the oven. Harvard University, on the other hand, is not especially tightly coupled, but is complex. A change in US student Visa policy; or a new government scheme to fund research; or the appearance of a fashionable book in economic’s, or physics, or anthropology; or an internecine academic row – could have unpredictable consequences for Harvard and trigger a range of unexpected responses, but none will spiral out of control quickly enough to destroy the University all together.”
In complex systems, there are many different ways for things to go wrong. In tightly coupled systems the consequences of something going wrong proliferate throughout the system quickly.
Put simply, tightly coupled systems are susceptible to the domino effect.