Hello @krilson,
Welcome to the forum!
Because of failed audits. Please, check your logs and search for GET_AUDIT and failed to see a reason. Also, the audit will be treated as failed if you have GET_AUDIT and download started, but not downloaded at the end. And of course, if audited piece is exist but corrupted, the audit will be treated as failed too. If audit score fall below 0.6 (60% on the dashboard), the node will be disqualified.