For about a decade (in many versions of Bridge for Mac os - from CS2 on up to the latest version) I've been observing that bridge gets really confused when you browse through the files in a stack.
Sometimes things go well, but often if you play through the stack and pause part way through, then click - the whole stack sort of explodes, and Bridge looses its grasp on weather the stack is open or closed. And even if you use the Colapse All Stacks menu item, the problem comes back when you click on that stack, or arrow past it with the arrow keys. Please see the video.
The problem has gotten Much better in recent versions, but it's not gone completely.
The only way to fix a broken stack is to select it, then upgroup from stack, then make a new stack out of it. Bridge is an awesome tool... please fix this bug.