There are all kinds of bugs that go after roots especially trying to grow trees in pots and even more if your trying to start something is indoors. It's like the natural predators and bio are out of whack and the bad can get multiplied quick if the manipulated environment hits right. Had those stupid little gnats go crazy on some nuts I was starting indoors a couple years ago, have had same but not as bad in pots outside. A little to much or not enough water sets that stuff off sometimes too.