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.