It is the first time happening to me. I was softbanned a couple of hours ago from getting pokestops (try again later). Then got unbanned, so started using necrobot, just for pokestops, after 20 minutes i got softbanned again. And after like 2-3 tries from the bot's unbanning attemps i got unbanned. Then after 5 minutes i got softbanned again lol. I'm wondering why happened that fast. I read that there was possibly a max amount of pokestops to rob per day, but if it is that so, then i shouldn't have been unbanned 2 times in the same day with that kind of softban. Well, just wanted to share this.
EDIT: After 30+ minutes of using Necro/Pogofeeder for sniping (because i could still catch pokemon) i'm unbanned again. Not sure if it has something to do. The bot was just trying to unban all the time without success, and sniping pokemon. And after 20 minutes or so, got softbanned again. After 15+ minutes, got unbanned again.