A couple people have mentioned getting trade banned and not knowing why.
So unless you track trades in excel, or whatever, then it's hard to say what could have been the reason. If it's not 4 copies of the same game, going over the $500 cash limit, or going over the $2k trade credit limit, it may have just been somebody looking into your account and not liking what they saw (if you're a regular trader).
I used to when I was trading every month, but I traded ~$1900 last September (all trade credit, tracked with excel), then ~$100 this January, and then nothing until today. Never went over 2k or went with cash, so I guess somebody just didn't like what they see last September?