"We {company owners/founders} are excited to announce that {company} is partnering with {venture capital firm} to take {company to the next level}. {company owners/founders} will be moving to the board of directors and a new CEO is coming aboard. It's a very exciting time for {company}."
Received a few of those emails in my time... it's always bad news and might as well get your resume together right then.
I’ve resigned twice in my career following that type of communication. Both were smallish startups outperforming the other company, which then acquired them and proceeded to turn everything to shit.