It looks like it was because of stale product data in shopping
>We believe that having a commercial relationship with merchants will encourage them to keep their product information fresh and up to date. Higher quality data—whether it’s accurate prices, the latest offers or product availability—should mean better shopping results for users, which in turn should create higher quality traffic for merchants.
This makes sense. I stopped using google shopping a long time ago precisely because it seemed like so many of the results were junk for whatever reason (staleness is a likely reason). "Junk" meaning the search result did not lead to an option to actually buy the product I was searching for. I haven't tried it since then, so I can't comment on if it's actually better now or if it's due to these changes.
>We believe that having a commercial relationship with merchants will encourage them to keep their product information fresh and up to date. Higher quality data—whether it’s accurate prices, the latest offers or product availability—should mean better shopping results for users, which in turn should create higher quality traffic for merchants.