that would be because you most likely had the dvd plugged into the v+ box then one from V+ up to the tv. When the dvd switched on the cablebox prioritised that signal and let it through the chain. In your TV there will be auto switch for both scart and HDMI but it will prioritise HDMI every time you switch on the DVD meaning the dvd wont flick over the input anymore.
One thing to try is to put the v+ box in standby then grabbing the television remote and switching the set from the hdmi input to the analogue tuner. Next switch on the v+ box this will make the set flick over to hdmi, when you switch off the v+ box the set should return to analogue and from there when you switch on the dvd it should auto change to your scart input et voilla
this is sound in theory although some sets wont work like this and some cableboxes always send signal through the scart regardless of whether it is in standby or operational