I am pretty sure people are immune to AFK reports in the beginning of the BG but I am not 100% sure.
It does sound like you were trolled. I used to do the same thing in BGs when I was younger since watching people flood out of the battleground was amusing in Alterac Valley. Report people that send what are usually emotes saying something such as “[Player name] has reported you AFK. Type /AFK now or you will be removed.” as I believe this is against the rules.
If you were reported AFK enough for the flagging system to trigger, you will see a debuff on your screen with a sheep icon, and if you don’t engage in combat before it expires, it turns into a debuff with a cow icon that will kick you from the match if you let it run out. However, if numerous people report you, then the countdown accelerates and can instantly expire with enough reports. I’m sure you know that but this information might be useful in the event that someone else runs into a similar problem and doesn’t know how the AFK flagging system works in battlegrounds.