Is this perhaps a Yahoo account? They are the only provider I know of that randomly moves mail for folders other than the inbox to spam.

No, it is a comcast acct. I think it has to do with Thuderbird. If I go to Xfinity, the email is where it is supposed to be. Thunderbird is moving the email.

By any chance have you got the 'Spam' column header enabled ? If you are selecting an email but accidentally clicking on the greyed out junk icon then you are marking email as junk. It would explain why some emails suddenly go to junk when you click on the email in Message List.