It's possible it was something else. But we can definitely say 2 things:
1. Valve bans for open plugin use now.
2. Moose was high-profile enough to create questions about what he was banned for.
A logical conclusion was that they banned for OpenPlugin, and picked somebody that would make enough of a ruckus among the community that people may preemptively disable it.
So it's a warning shot. If it was something else, so be it. But it's likely that they banned for this exact purpose.