I think once a post passes 10 upvotes, flags are no longer relevant. My guess would be that the posts are gaining upvotes fast enough (15+ upvotes a minute) that HN's anti-spam triggers are going off.

Possibly true, but that would mean the flag system can be abused to get rid of any articles you don't like. If this is the case that should definitely be looked into.

If it's human intervention then I really don't understand why it's happening: It's quite clear a lot of people want to discuss those messages on the social media accounts of sendgrid.