The short and simple answer is that you should have tried to ram them instead, but you didn't necessarily break any rule. The "what's brevity mean" answer goes like this...
In a perfect server, where everyone knows and follows the rules all the time, and everyone is perfectly reasonable, rational and has plenty of time to make decisions:
For a situation like what you've described, your best option is to ram them to try to flip their hog and/or run over the gunner before he gets in (if you're close enough). Sounds like chase killing, I know, but see
rule 8.2 in the chase killing section of the rules.
If you're too far away to ram them before the gunner is in, then you probably have time to swerve out of their path. If swerving is not for you, continue forward and when the gunner is in the hog and ready to shoot (you should be close enough by now), jump out and nade them.
When you're on foot and find yourself with two enemies on foot (2v1), you can attack them and try to kill one so that it's even (1v1). You can jump out of your lone hog to nade a loaded enemy hog when they're close and dangerous. You kind of combined the two and the result was right on the line of fair and legal. Before they teamed up, the enemy hog was a lone hog, so off limits. The future gunner was presumably not attacking or even looking at you (yet), and not a threat. So in that nanosecond of time, they were not fair game for you, but you jumped out to kill them anyway.
What you did wasn't really wrong. It was a quick decision and things would have been perfect if you'd waited half a second, and that's not a big deal. I personally would not have said anything about it. It looked fishy to Effect, and his quick decision was to "tsk" and let it go (which is pretty good considering all of it). The heat of the moment isn't the best time for quick decisions, but I hope this one worked out well enough.
Hopefully I've given you enough info to make speedy decision making even harder when it comes time, haha. If you have any other questions about it, let me know. It's always my pleasure to tell people about all the legal kills they can get