Why is Gmail marking emails as SPAM?
This is because Gmail users (yourself or others) have marked emails from you as spam either deliberately or accidentally.
Gmail automatically helps identify spam and suspicious emails by detecting viruses, finding patterns across messages, and learning from what Gmail users like you commonly mark as spam or phishing.
If you click your Spam label and open one of the messages, you’ll see a warning label at the top with a brief explanation about why that particular message was placed in Spam. If we think something is suspicious, we’ll mark the message with a warning label in your inbox for you.
You (or someone else) previously marked these messages as spam by clicking the “Report spam” or “Report phishing” button. Both actions will send the message to your Spam folder and remove it from your Inbox.
What you need to know
After you report spam for several messages from the same sender, our system will learn from your behavior and might continue placing that sender’s messages to Spam even if you don’t specifically report them.
Also know that you can create filters to route certain messages from Spam to your Inbox, or route spam messages to your Trash. We’ll let you know whenever your filters caused a message that our system identified as spam to be moved to your Inbox; if you want those messages to stay in Spam, consider changing your filters.
Actions you can take
If you don’t want the message to be in Spam, click the “Not spam” button to move it into your Inbox. If you mistakenly marked it as a phishing scam, please click the down arrow next to “Reply” at the top-right of the message pane, and select “Report not phishing” to let us know that the message is legitimate. By reporting that a message is not spam or phishing, you’ll help teach Gmail’s system what to do when it sees a similar message in the future.
Source: Gmail Spam and Suspicious Emails