To state it broadly, the major reason for increase in a website's spam score is creating backlinks in spammy or irrelevant sites. These sites violate the Google guidelines, so if you continue creating backlinks on them, you may action from Google too. If you dig deeper, you'll know that Moz has 27 signals or attributes which it checks before assigning spam score to a website. Some of the major attributes are:

1. Domain name length
2. Domain name contains numerals
3. Google Tag Manager is not present
4. TLD connected with spammy sites