That's not mutually exclusive with "someone on the sales team uploaded the entire customer list for sales purposes, not realizing the privacy implications".
>more frequently than databreaches.
You're fighting against both hanlon's razor and occam's razor here. The OP states the leak came from Apollo, and as other commenters have noted, Apollo specifically has a "Contributor Network" that shares email lists with other companies, and isn't well documented. It's not hard to imagine how this was done unintentionally. On the other hand there's no evidence to suggest this was done intentionally, other generic cynicism of "businesses do things in the business's interest" or whatever.
What is more likely? Everyone at an organization's IT, sales and data protection department is incapable of doing their job, or someone doesn't give a damn, calculating, that preventing such things from happening costs too much?