I was attacking my farm account then it auto shield up from my attacking. Anyone know why this happened?

Possibly it was a game update, occasionally the developers make an update on the server side which restarts the game and will deploy a shield automatically to all players for a short period. I am not aware of any other way that a shield can be added automatically.

The first time you attack your farm, part of the storyline is that Ada pops a shield for you on the first attack.

I'm not protected by auto shield from the first attack. I have to manually use the shield. some of my mates got auto shield after the first attack on the same day. they also didn't know how this happened. they told me that, they receive an alarm, open their base,something like a box was in front of the gate, they click it and it shows to radar & just click on it, suddenly their base are shield. 24 hours auto shield. clueless.