Well ... you were engaging ... the other guy was defending himself. Rules state that the guy has a right to defend himself. You don't have to make any statement to defend yourself. Sorry ... but you declared your intention to attack in support of the LSF ... makes you the aggressor and the other guy the defender.
Even if you didn't declare your intent of joining the battle on the LSF side, by hanging around for some time you are joining the battle ... expect to get hurt. That counts for "innocent observers" as well. If you stick your nose into a dogfight, you can get hurt.
Edit: That said, it's a good form not to kill a new participant to a fight right when he appears, especially when it's likely that he's typing. I want to have fun fighting, not get a blue message at all costs. If the opponent isn't actively fighting, I usually fire a volley to get his attention and then wait for him to act ...