While we would rather racers concentrated on hogs at the start of the map in public servers, there is no rule against nading... especially if it is a 2 on 1 situation like you imply here. If they were that close to jumping into a rocket hog you could have held off another nanosecond until they were IN the hog, while at the same time moving towards a hog yourself. Once they were in a rocket hog together you would almost surely have died so your pre-emptive strike is totally understandable.
Your other option would be to hide until they had headed out of the base. Some spawn areas on don't really allow much of that.
Short answer : You did nothing wrong based on what you describe