This could be for a number of reasons. No major tld exposes their algorithm so it is very difficult to say exactly. But it is usually for one of the following reasons:
- Someone clicked "spam" on an email that you sent
- You received a high number of bounces
- You sent to a honeypot, or spam track email address
- Your content contains flagged words
- Your content, or an identifying mark in your content has been flagged
- You tried to send to much volume, to fast, to too many of the same provider's email addresses
- Your new domain ownership did not register properly, and your still being ranked based on the prior owner's behavior
- Your email automation technology is not coding its outgoing mail properly
- Your email automation provider is using a bad tracking pixel
- Someone severely negatively impacted your Shared IP
- Your sending IP is moderate to negatively ranked