I think your most probably correct with poor manufacture being most likely reason for the failures.
Manufactured in built stress, non uniform thermal expansion, induced shock, tensile loading, shear stress, point loading, compressive loading, torsional stress and so on are all things that could, however, cause a poorly manufactured windscreen to burst.
Both uneven road surfaces and jacking the van, subjects its chassis and shell to stress. Crucially with stress, you get strain (Hookes Law).
Sometimes when driving, its unavoidable to do the twist, but as I'm planning on keeping my van a while and quite fond of the windscreen I already have, I'll just er on the side of mechanical sympathy, when changing wheels and tyres meantime.
Also keen that my suspension struts remain where they're designed to be, ie......on the underside of the bonnet. So I'll take your word on the Dukes of Hazard style experiment. Plus, heights I'm ok with. Falling from them.....less so.