I don't think the search bots are in the habit of just 'stumbling,' they follow very specific algorithms
In general, they are simply following all links they find on different web pages. If they find a link to a cloud drive and can follow it, it means the access to the content wasn't properly secured. Search bots don't implement any algorithms allowing them to find hidden and protected content. Moreover, there are simple means to prevent them scanning a specific web site.
There is one very unpleasant circumstance, though. First, if you use Google Drive for content sharing purposes, you can either give someone passport-protected access to it or just generate a web link allowing anyone to access it. The first method requires the user to have a Google account (and not all people have it), so the second one is often used. It's OK if you don't put this link on a publicly accessible web site. However, here comes the second issue. Google automatically scans mailboxes of its users for different purposes. For example, it you book your hotel on the Booking.com web site and it sends a confirmation to your Google mailbox, you can open the web mail interface and see that the confirmation letter has special buttons attached to it. In addition, the location of the hotel will be automatically shown on Google maps of the area you open. So combining those two issues, you can have web links from your messages leaked to Google bots. To its search bot, in the first place. While it might be done unintentionally, it still has grave consequences.
I suspect that the publisher simply shared a folder on its Google Drive for everyone and then sent a link to someone owning a Google mailbox. The link leaked to the search bot, and voila! The content can be accessed by anyone on the Internet.
The moral of the story is, always secure others' access to your cloud data with a password and never keep unencrypted data there even if you give the access to no one. You never know who might become interested in your data, a Google's system administrator or a CIA employee, whoever...