There are some very obvious reasons why users get red tagged or flagged, and with the update by theymos on the trust system, we now understand that a user can only be flagged for being a proven scammer, and not for some other offences. But for the newbies who probably aren't aware of some of offences that can get them red tagged on the forum, I'll love to bring some of them to their notice.
1)
Do not default on loans/do not request for no collateral loan unless the loaner says so/do not request for loans when you have no means to pay back: Many users have had their accounts tagged due to the fact that they defaulted on their loan agreement, if you know you have no means to payback a loan, do not request for one, also as a newbie you can simply get red tagged for just asking for a no collateral loan(no one will give a newbie that)
2)
Avoid renting out your account/borrowing your account to friends: We have seen issues like this in the past when users after being caught with plagiarised posts on their account claim they were not the ones handling the account at the moment those posts were made. The user you give your account to, can do shady things that will get your account tagged, you account is your private property.
3)
Do not advertise or market a website you aren't sure of its legitimacy: Some users brings some shady website to the notice of others, and a few times they are not even aware that those websites are shady sites, this leaves dt members with no choice than to tag those accounts. We had an issue sometime of a newbie posting websites where one can recover lost btc by paying a particular amount of money, that user was tagged for promoting a scam website.
4)
Avoid account buying and selling: Going through many ratings left by dt members, I think this one tops the list: "Account buyers and sellers are not to be trusted", many users here started from the bottom to get where they are today, buying an account is fraud and will get you red tagged, the original owner could even have plagiarised posts, could later come back claiming the account was hacked, or you could even pay and not get the account in the end.
5)
Spamming pm's begging for merits/adverting a product: If you want to earn merits, make good posts, it will definitely be seen by sources and other members, and even if you must pm a merit source, let it be to review your post history, do not beg them for merits. Also the services board is there for users to advertise, if a user hasn't requested for a discussion on a product in their pm, do not send one, it can get you into trouble.
Edit: you can learn more from this threads:
Trust flagsLoyceV's Beginners guide to correct use of the Trust system