Slow down a bit. The system is quite ingenious. As long as the vehicle is travelling at less than 50mph the windows stay open.Once above this speed, they close automatically to mnaintain streamlining.. A bit like a posh car's, rear spoiler popping up above a certain speed..
I would say the rear window hinges are worn, but then I've never see inside a 3 door disco but do know how similar windows work on other cars which blow closed with the wind , been there .