I'm not sure exactly how these seats are put together or what mechanism keeps them in the upright position, but if I were a betting man, I would say that the teeth that interlock to hold the seat upright have become worn or stripped. Think of a bike chain and the teeth that allow the chain to be grabbed by the sprocket. May be time for either a new seat or at least a new set of "teeth" to install into the intersection of top and bottom seat. I really wish some of these more informed guys would speak up on this, as I can see how this could be a huge safety problem! Call your local dealer and speak to someone in the service dept or maybe call a Volvo mechanic in your area and explain your problem. Maybe they can give a better response. Hope I've helped you at all!