My D1a did that a few years ago. Parked on a rural property, backed up against the shed and I know that no-one got near it. Overnight it just completely shattered. The tint held it all together, thankfully. No signs of impact, nothing disturbed or out of place. Took a couple of goes to get the correct glass fitted. The first one had the high mount brake light mask upside down! Glad the insurance covered that whole debacle...
Had a windscreen on the old Volvo wagon crack for no reason,  too. No rust or impact. Just suicide...
Anyway, tough luck right there. I think it's just one of those things that can happen randomly. If there's no starred propagation point (external impact or point load) then it's likely just internal stresses getting too much. Bugga.
				
			 
			
		 
			
				
			
			
				DiscoClax
'94 D1 3dr Aegean Blue - 300ci stroker RV8, 4HP24 & Compushift, usual bar-work, various APT gear, 235/85 M/Ts, 3deg arms, Detroit lockers, $$$$, etc.
'08 RRS TDV8 Rimini Red - 285/60R18 Falken AT3Ws, Rock slider-steps, APT full under-protection, Mitch Hitch, Tradesman rack, Traxide DBS, Gap IID 
			
			
		 
	
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