Volvo probably found a problem that can occur at higher miles of use in extended testing. Once they identified the problem then they realized 0-20 would prevent it, at least for a longer period of time. So they made the switch in order to prevent/delay the problem for customers beyond the expected life of the vehicle.
And example that comes to mind is the early Ford Y blocks. They used an oiling system that worked perfect on paper. But once out in the real world folks would delay and skip oil changes. The non-detergent oil at the time would not clean up the gunk as well and eventually the oil passages the the furthest valves from the start point would get clogged on each bank. This was in the 1950s so the solution was to just buy another car or pinch the end of the oil line to create more pressure. But if Ford wanted a different real solution they might have developed a high detergent oil as a rebuilt motor today does not suffer these problems due to better oil.
Just a story there. I would guess either something where oil pressure is needed at start up where the engine is still cold or possibly a drain return issue where 30w ended up being too thick (like maybe thicker oil was suffering from more coking in the turbo with 30w). Because it was not at launch it was likely due to a discovered problem. Again, that's all speculation, but I think it's fairly rational. It would explain why even at the dealer level we can't say "oh this happens" because we never saw it. If you run 5w-30 for 300k miles you can let us know, but I'd just run the 0w-20 of the correct specification and know it's been determined to be the best for your motor.
