When I joined the game I saw you running away from a cannon that had been blown up and you said you griefed your team mates cannon. Then you ran to your team mates new cannon and blew that one up too.
With the team mates auto cannon you should've used /report. Instead, it appeared as if you broke it and caused it to blow up, then you claimed to grief it. I'm not sure exactly what happened because I wasn't there to see it. However I can say with certainty that you ran up to his other cannon and tried to fire it without considering whether or not he was ok with you using his cannon and that was before he was even finished building. By doing this you griefed his second cannon.
If you did one or the other I would have simply warned you and left it at that, even though warnings are meant to inform players of the rules you already know. If you were more considerate towards your team mate and conscious of the rules and how to abide by them you could have avoided this ban.