By the sound of things it’s more like nobody wanted anything to do with the major-party incumbent. Duverger’s law is about how there tend to be two parties. Three and one are equally unstable. When a race becomes a total rout, like a 30-point spread, that dominance can be seen as a power vacuum.
… also, Sri Lanka has ranked ballots. It’s not a Plurality voting system. They have an automatic runoff. That’s one of the more obvious fixes that allows people to even consider supporting a third party, without playing Russian roulette against their own foot.
People acting in their best interests is how it happens. It’s an electorate avoiding splits. Given the system you’re voting under - you should vote for someone who has a chance of winning. Otherwise you might write-in some special favorite candidate that no other human being cares about, and accomplish literally nothing. Voting for a third party with single-digit support is not much better.
People voting against their own interests would be… not bothering to write in a second preference. It is the same fuckup: someone who cannot imagine their very favorite guy losing.