Hi there!
Sam from Moz's Help Team here! It looks like PlanetHurf has sent you to a great resource about why the Domain Authority might drop for a particular site - do let me know if you have any follow up questions about that.
The Spam Score isn't entirely dependent on backlinks. We also calculate this score by analysing your site itself (the URL that you enter into Open Site Explorer) You can click directly on the Spam Score from within Open Site Explorer, and you'll see a list of reasons we've assigned a site a particular score - for instance with https://moz.com/researchtools/ose/spam-analysis/flags?subdomain=truckcare.com, you'll see some of the markers are 'Thin Content', 'Low Number of Pages Found', and 'No Contact Info'.
The general rule we have at Moz is that you shouldn't be concerned about anything that has a spam score under 8. Additionally, just because our tool has assigned a site a specific spam score does not mean it is impacting the site in any way. These are merely suggestions of things you might want to check out. If you haven't determined yourself that they are spammy links, then there is no action or further concern required on your part - looking into backlinks from sites with spam scores of 8 or higher would definitely be a good idea, just to ensure that you aren't being linked to from a site that is actually spammy.
If you'd like to learn more about the Spam Score in general, here are some super helpful resources:
Article: Spam Score: Moz's New Metric to Measure Penalization Risk
Video: Understanding and Applying Moz's Spam Score Metric - Whiteboard Friday
Let us know if we can help with anything else!