It might haven't been your intention to do plagiarism, but you are supposed to mention the source, which would specify that the content is not your own but is copied from somewhere else.
It would be a little harsh if that was the reason for the ban, we would have to wait for a mod to clarify.
I interprete Pliagarism as an attempt to copy someone else's work and pass it off as your creation, in order to get acknowledgement or some other form of benefits, or to boost your post count.