Under the Finder, when you launch an application from HyperCard then quit that application, the "resume" system message is sent and you return to HyperCard instead of to the Finder.
MultiFinder, on the other hand, patches the "launch" ROM call and handles it before HyperCard is passed the launch information. And HyperCard is not informed when you quit from the application. Consequently, you don't always return to HyperCard when you quit from the application that was launched from HyperCard.