In reply to Siward:
A wild guess. On my Ducato Campervan, if the sliding door opens fully it engages a catch so that you have to pull the handle outwards before pulling shut. This is quite annoying, particularly if you are parked so that the door slides open of its own accord. So maybe it's been rigged to prevent that. Also, if it's a campervan and the rear window opens outwards and you forget to shut it, the sliding doors slams into it and breaks it.
In fact I think I might do the same!