I think your display mode somehow was accidentally changed. If someone links to a topic in a different display mode, it for some reason changes your preferences.
At the top of any topic, click the Options button on the right. There are three display modes listed: Outline, Standard, and Linear+. Outline is the one you probably don't like. Standard lists the posts within a topic from oldest to newest. Linear+ is the same as standard, but includes the first post of the topic at the top of the first and all subsequent pages.
Our default is Linear+, but you're obviously able to choose whichever you want.