One event sets things up for another event, and so on. (As an explanation of how little things lead to big problems.) I kept spending more and more money until I was broke. You know how one thing leads to another.He bought a car, then a house, then a boat. One thing leads to another.
an event or activity results in another that you have usually not planned I agreed to help him paint his house and one thing led to another until I ended up helping him fix up his kitchen.
if one things leads to another, a series of events happen, each one caused by the previous one (never in continuous tenses) I only asked him in for a coffee, but one thing led to another and we ended up in bed together.People don't usually decide to become spies. They agree to do someone a favour and one thing leads to another.