this is a bug that's been around since GBG started, but it's just extremely rare. As far as I know there's nothing they or you did to cause it, except maybe two guilds fighting very very fast. I've had it happen once this season to us and once last season to another guild, but both seasons have been extremely fast paced seasons between the top three guilds on our server. I first saw this within a few months of GBG first coming out..
there's nothing you can do other than just taking the sector over and over to halve the encounters, or if they take it (they'll get to keep that head start) then it'll of course clear like normal.