Here are some of the explanations you might see:
1. "You previously marked messages from info@example.com as spam."
2. "You clicked 'Report spam' for this message."
3. "It's written in a different language than your messages typically use."
4. "It contains content that's typically used in spam messages."
5. "It's similar to messages that were detected by our spam filters."
6. "Many people marked similar messages as spam."
7. "We've found that lots of messages from info@example.com are spam."
8. "Be careful with this message. Our systems couldn't verify that this message was really sent by amazon.com. You might want to avoid clicking links or replying with personal information."
9. [Phishing] "Be careful with this message. Similar messages were used to steal people's personal information. Unless you trust the sender, don't click links or reply with personal information."
#4 and #5 are the most common explanations and they're rather vague. It's interesting to notice that the messages written "in a different language than your messages typically use" could be flagged as spam, but this shouldn't be the only explanation.
"We hope that this is not only interesting, but also helps you learn about scams and other harmful messages that Gmail filters out. Whether you prefer to leave your spam folder untouched or do some educational digging, the information will be there for you. And if you're interested in learning more, check out our new series of spam articles in the Gmail help center," informs Google.
{ Thanks, Venkat. }