Applications that are not active are supposed to go to sleep unless specifically designed not to. When you start the application again, it just wakes it up, minimizing start up times. If there are too many background applications taking up memory, Android will begin killing them off.One problem with Android seems to be that it likes to keep all programs running.
Either this build didn't do these things it was supposed to, or the applications you were running were designed to keep running even while in the background for unknown reasons.
Ah. Okay. Or maybe this is some CM7 setting that needs to be tweaked.